allozaur/feat/chat form contenteditable (#26717)

* feat: Add contenteditable tokenizer for badge/code-chip chat input

* feat: Add source-space undo/redo history for the rich input

* feat: Split text glued to a closing code fence onto its own line

* feat: Add ChatFormContenteditable rich input renderer

* feat : wire the contenteditable into ChatForm with auto-switch gating

tools/ui/src/lib/components/app/chat/ChatForm/ChatForm.svelte

@@ -2,6 +2,7 @@
 	import {
 		ChatAttachmentsList,
 		ChatFormActions,
+		ChatFormContenteditable,
 		ChatFormFileInputInvisible,
 		ChatFormMcpResourcesList,
 		ChatFormPickers,
@@ -46,10 +47,12 @@
 	} from '$lib/types';
 	import {
 		buildMentionInsertion,
+		containsCodeSpan,
+		containsFileMentionLink,
 		findCommandToken,
 		findMentionToken,
 		isIMEComposing,
-		mentionLinkEndingAt,
+		isOffsetInCodeBlock,
 		parseClipboardContent,
 		uuid
 	} from '$lib/utils';
@@ -109,12 +112,28 @@
 		onValueChange
 	}: Props = $props();
 
+	// Component References
+	// Shared handle of the two input renderers (textarea + contenteditable).
+	type ChatInputHandle = {
+		focus(): void;
+		resetHeight(): void;
+		getElement(): HTMLElement | undefined;
+		getCaretOffset(): number;
+		setCaretOffset(offset: number): void;
+	};
+
 	let audioRecorder: AudioRecorder | undefined;
 	let chatFormActionsRef: ChatFormActions | undefined = $state(undefined);
 	let fileInputRef: ChatFormFileInputInvisible | undefined = $state(undefined);
 	let pickersRef: { handleKeydown: (event: KeyboardEvent) => boolean } | undefined =
 		$state(undefined);
-	let inputRef: ChatFormTextarea | undefined = $state(undefined);
+	let inputRef: ChatInputHandle | undefined = $state(undefined);
+
+	// Render-mode gate: the plain textarea by default, the contenteditable
+	// while the buffer carries a `file://` mention link or a complete code
+	// span (badges and code chips need a DOM the textarea cannot provide).
+	// Demotes back once neither remains.
+	let useContenteditable = $state(false);
 
 	// Audio Recording State
 	let isRecording = $state(false);
@@ -201,6 +220,34 @@
 	);
 	let canSubmit = $derived(value.trim().length > 0 || hasAttachments);
 
+	// Caret offset restored after a renderer swap. Callers that mutate
+	// `value` themselves (e.g. the mention picker) pin the target offset
+	// BEFORE the assignment; otherwise the swap effect snapshots the
+	// current caret.
+	let pendingCaretOffset = 0;
+	let caretOffsetPinned = false;
+
+	function queueCaretRestore() {
+		queueMicrotask(() => {
+			inputRef?.focus();
+			inputRef?.setCaretOffset(pendingCaretOffset);
+			caretOffsetPinned = false;
+		});
+	}
+
+	$effect(() => {
+		const wantContenteditable =
+			containsFileMentionLink(value ?? '') || containsCodeSpan(value ?? '');
+		if (useContenteditable === wantContenteditable) return;
+
+		if (!caretOffsetPinned) {
+			pendingCaretOffset = inputRef?.getCaretOffset() ?? (value ?? '').length;
+		}
+
+		useContenteditable = wantContenteditable;
+		queueCaretRestore();
+	});
+
 	onMount(() => {
 		recordingSupported = isAudioRecordingSupported();
 		audioRecorder = new AudioRecorder();
@@ -252,25 +299,19 @@
 			return;
 		}
 
-		// Backspace at a mention link's end deletes the whole token at once.
-		if (event.key === KeyboardKey.BACKSPACE && !event.ctrlKey && !event.metaKey && !event.altKey) {
-			const el = inputRef?.getElement();
-			if (el instanceof HTMLTextAreaElement && el.selectionStart === el.selectionEnd) {
-				const link = mentionLinkEndingAt(value, el.selectionStart);
-				if (link) {
-					event.preventDefault();
-					value = value.slice(0, link.start) + value.slice(link.end);
-					onValueChange?.(value);
-					queueMicrotask(() => inputRef?.setCaretOffset(link.start));
-					return;
-				}
-			}
-		}
-
 		if (event.key === KeyboardKey.ENTER && !event.shiftKey && !isIMEComposing(event)) {
 			const isModifier = event.ctrlKey || event.metaKey;
 			const sendOnEnter = currentConfig.sendOnEnter !== false;
 
+			// Caret inside a fenced code block (closed, or still open
+			// while being typed): Enter adds a line, never submits. The
+			// contenteditable consumes this case locally; this gate
+			// covers the plain textarea, where skipping submit lets the
+			// native newline through.
+			if (!isModifier && isOffsetInCodeBlock(value ?? '', inputRef?.getCaretOffset() ?? 0)) {
+				return;
+			}
+
 			if (sendOnEnter || isModifier) {
 				event.preventDefault();
 
@@ -444,14 +485,20 @@
 		const built = buildMentionInsertion(entry, value, token);
 		if (!built) return;
 
+		// Pin the post-insertion caret BEFORE the swap effect runs;
+		// otherwise the effect clobbers it with the textarea's selection
+		// at promotion time (browser-dependent: usually reset to 0).
+		pendingCaretOffset = built.caretOffset;
+		caretOffsetPinned = true;
+
 		value = built.newValue;
 		onValueChange?.(built.newValue);
 
-		// bind:value applies on the next microtask; restore the caret after.
+		// Already in contenteditable mode: no renderer flip, so the swap
-		queueMicrotask(() => {
+		// effect's caret restore never runs.
-			inputRef?.focus();
+		if (useContenteditable) {
-			inputRef?.setCaretOffset(built.caretOffset);
+			queueCaretRestore();
-		});
+		}
 	}
 
 	async function handleMicClick() {
@@ -541,19 +588,35 @@
 		<div
 			class="flex-column relative min-h-12 items-center rounded-4xl md:rounded-3xl py-2 pb-2.25 shadow-sm transition-all focus-within:shadow-md md:py-3!"
 		>
-			<ChatFormTextarea
+			{#if useContenteditable}
-				class="px-5 py-1.5 md:pt-0"
+				<ChatFormContenteditable
-				bind:this={inputRef}
+					class="px-5 py-1.5 md:pt-0 mb-0.5"
-				bind:value
+					bind:this={inputRef}
-				onKeydown={handleKeydown}
+					bind:value
-				onInput={() => {
+					onKeydown={handleKeydown}
-					pickers.handleInput();
+					onInput={() => {
-					onValueChange?.(value);
+						pickers.handleInput();
-				}}
+						onValueChange?.(value);
-				onPaste={handlePaste}
+					}}
-				{disabled}
+					onPaste={handlePaste}
-				{placeholder}
+					{disabled}
-			/>
+					{placeholder}
+				/>
+			{:else}
+				<ChatFormTextarea
+					class="px-5 py-1.5 md:pt-0"
+					bind:this={inputRef}
+					bind:value
+					onKeydown={handleKeydown}
+					onInput={() => {
+						pickers.handleInput();
+						onValueChange?.(value);
+					}}
+					onPaste={handlePaste}
+					{disabled}
+					{placeholder}
+				/>
+			{/if}
 
 			{#if mcpHasResourceAttachments()}
 				<ChatFormMcpResourcesList

tools/ui/src/lib/components/app/chat/ChatForm/ChatFormContenteditable.svelte

@@ -0,0 +1,788 @@
+<script lang="ts">
+	import { onDestroy, onMount, untrack } from 'svelte';
+	import { mode } from 'mode-watcher';
+	import githubDarkCss from 'highlight.js/styles/github-dark.css?inline';
+	import githubLightCss from 'highlight.js/styles/github.css?inline';
+	import { isMobile } from '$lib/stores/viewport.svelte';
+	import { ColorMode } from '$lib/enums';
+	import { TRIM_LEADING_PADDING_REGEX, TRIM_TRAILING_PADDING_REGEX } from '$lib/constants';
+	import {
+		badgeAwareWordJump,
+		buildFragment,
+		domMatchesTokens,
+		highlightCode,
+		isIMEComposing,
+		isOffsetInCodeBlock,
+		leadingBadgeEdgeOffset,
+		rangeToTextOffset,
+		serializeContent,
+		SourceHistory,
+		stripBlockBoundaryLineBreaks,
+		syncCodeBlockHatches,
+		tokenizeContent,
+		textOffsetToRange
+	} from '$lib/utils';
+	import type { ContentToken, SourceHistoryEntry } from '$lib/utils';
+
+	interface Props {
+		class?: string;
+		disabled?: boolean;
+		onInput?: () => void;
+		onKeydown?: (event: KeyboardEvent) => void;
+		onPaste?: (event: ClipboardEvent) => void;
+		placeholder?: string;
+		value?: string;
+	}
+
+	let {
+		class: className = '',
+		disabled = false,
+		onInput,
+		onKeydown,
+		onPaste,
+		placeholder = 'Ask anything...',
+		value = $bindable('')
+	}: Props = $props();
+
+	let rootElement: HTMLDivElement | undefined = $state();
+	let lastEmittedValue = '';
+	let isComposing = $state(false);
+
+	// Undo/redo in source space: the imperative token rebuilds destroy the
+	// browser's native undo stack.
+	const history = new SourceHistory();
+
+	// Browsers disagree on what an empty contenteditable contains (`<br>`,
+	// `<div><br></div>`, or nothing), so emptiness is decided by the
+	// serialized source, not the DOM shape.
+	function syncEmptyState(serialized?: string) {
+		if (!rootElement) return;
+		const source = serialized ?? serializeContent(rootElement);
+		rootElement.dataset.empty = source.length === 0 ? 'true' : 'false';
+	}
+
+	function renderTokens(tokens: ContentToken[]) {
+		if (!rootElement) return;
+
+		const caret = rangeToTextOffset(rootElement, safeRange());
+
+		// eslint-disable-next-line svelte/no-dom-manipulating -- the token layer is owned imperatively; Svelte renders only the contenteditable host, never its children
+		rootElement.replaceChildren(buildFragment(tokens));
+
+		syncCodeBlockHatches(rootElement);
+		highlightCodeBlocks(rootElement);
+
+		restoreCaret(caret);
+		resizeHeight();
+		syncEmptyState();
+	}
+
+	// Last highlighted source segment per block element - typing inside
+	// a block re-highlights only when the segment actually changed.
+	const highlightedSegments = new WeakMap<HTMLElement, string>();
+
+	const CODE_BLOCK_OPEN_RE = /^```([^\n`]*)\n/;
+
+	/**
+	 * Apply syntax highlighting to a code block element's CONTENT. The
+	 * fence lines stay plain text, and the blank padding that
+	 * `highlightCode` trims is re-added as plain text, so the element's
+	 * textContent stays byte-exact with the source segment. Replaces
+	 * the element's children - callers restore the caret afterwards.
+	 * Returns false when nothing changed.
+	 */
+	function highlightCodeBlockElement(el: HTMLElement): boolean {
+		const segment = el.textContent ?? '';
+		if (highlightedSegments.get(el) === segment) return false;
+
+		const open = CODE_BLOCK_OPEN_RE.exec(segment);
+		if (!open) return false;
+
+		const prefix = open[0];
+		const language = open[1].trim().split(/\s+/)[0] ?? '';
+		const content = segment.slice(prefix.length, -3);
+
+		const leading = content.match(TRIM_LEADING_PADDING_REGEX)?.[0] ?? '';
+		const trailing = content.match(TRIM_TRAILING_PADDING_REGEX)?.[0] ?? '';
+		const core = content.slice(leading.length, content.length - trailing.length);
+
+		// autoDetect off: re-guessing the language on every keystroke
+		// costs ~38ms a call and flickers while typing
+		const html = core ? highlightCode(core, language || 'text', false) : '';
+		const tpl = document.createElement('template');
+		tpl.innerHTML = html;
+
+		el.replaceChildren(
+			document.createTextNode(prefix + leading),
+			tpl.content.cloneNode(true),
+			document.createTextNode(trailing + '```')
+		);
+		highlightedSegments.set(el, segment);
+		return true;
+	}
+
+	function highlightCodeBlocks(root: HTMLElement) {
+		for (const el of root.querySelectorAll<HTMLElement>('code[data-code-token="block"]')) {
+			highlightCodeBlockElement(el);
+		}
+	}
+
+	/**
+	 * Re-highlight the code block the caret sits in after an edit.
+	 * Skipped when the block's segment is unchanged since its last
+	 * highlight, so edits outside blocks cost nothing.
+	 */
+	function rehighlightCaretCodeBlock() {
+		if (!rootElement) return;
+
+		const range = safeRange();
+		if (!range) return;
+
+		let node: Node | null = range.startContainer;
+		if (node === rootElement) {
+			node = rootElement.childNodes[range.startOffset - 1] ?? null;
+		}
+
+		while (node && node !== rootElement) {
+			if (node instanceof HTMLElement && node.dataset.codeToken === 'block') {
+				const caret = rangeToTextOffset(rootElement, range);
+				if (highlightCodeBlockElement(node)) {
+					restoreCaret(caret);
+				}
+				return;
+			}
+			node = node.parentNode;
+		}
+	}
+
+	/**
+	 * Is the caret inside a fenced code block region? Source-level
+	 * (not DOM-level) so the still-OPEN fence counts too: while the
+	 * user is typing a block, no closing ``` exists yet and the
+	 * buffer is plain text with no block element to find. Root-level
+	 * caret positions right at a closed block's edge (escape
+	 * hatches, element boundaries restored by `textOffsetToRange`)
+	 * resolve past the closing fence, so they count as OUTSIDE.
+	 */
+	function caretInCodeBlock(): boolean {
+		if (!rootElement) return false;
+
+		return isOffsetInCodeBlock(
+			serializeContent(rootElement),
+			rangeToTextOffset(rootElement, safeRange())
+		);
+	}
+
+	/**
+	 * hljs theme for the highlighted code blocks. Mirrors
+	 * SyntaxHighlightedCode.svelte: one shared style element
+	 * (deduped via the data attribute) swapped on mode change.
+	 */
+	function loadHighlightTheme(isDark: boolean) {
+		document.querySelectorAll('style[data-highlight-theme-preview]').forEach((s) => s.remove());
+
+		const style = document.createElement('style');
+		style.setAttribute('data-highlight-theme-preview', 'true');
+		style.textContent = isDark ? githubDarkCss : githubLightCss;
+
+		document.head.appendChild(style);
+	}
+
+	$effect(() => {
+		loadHighlightTheme(mode.current === ColorMode.DARK);
+	});
+
+	function safeRange(): Range | null {
+		if (!rootElement) return null;
+
+		const selection = window.getSelection();
+		if (!selection || selection.rangeCount === 0) return null;
+
+		const range = selection.getRangeAt(0);
+
+		if (!rootElement.contains(range.startContainer) || !rootElement.contains(range.endContainer)) {
+			return null;
+		}
+
+		return range;
+	}
+
+	function restoreCaret(offset: number, extend = false) {
+		if (!rootElement) return;
+
+		const target = textOffsetToRange(rootElement, offset);
+		const selection = window.getSelection();
+		if (!selection) return;
+
+		if (extend && selection.anchorNode) {
+			selection.setBaseAndExtent(
+				selection.anchorNode,
+				selection.anchorOffset,
+				target.startContainer,
+				target.startOffset
+			);
+			return;
+		}
+
+		selection.removeAllRanges();
+		selection.addRange(target);
+	}
+
+	function resizeHeight() {
+		if (!rootElement) return;
+		rootElement.style.height = 'auto';
+		rootElement.style.height = `${rootElement.scrollHeight}px`;
+	}
+
+	function recordHistory(newGroup: boolean) {
+		if (!rootElement) return;
+		history.push(
+			{ value: lastEmittedValue, caret: rangeToTextOffset(rootElement, safeRange()) },
+			Date.now(),
+			newGroup
+		);
+	}
+
+	/**
+	 * Re-emit the current markdown source value to the parent, then
+	 * reconcile the DOM against the token stream: when a code span
+	 * was just completed or broken, the token boundaries no longer
+	 * match the element structure and the DOM is rebuilt (caret
+	 * preserved through the source-offset mapping).
+	 */
+	function processInput(inputType?: string) {
+		if (isComposing || !rootElement) return;
+
+		syncEmptyState();
+		resizeHeight();
+
+		// Shift+Enter right after a code block leaves an all-newline
+		// text node (the fence's separator line plus Chromium's
+		// artificial end-of-buffer line break). Strip both so the caret
+		// lands on the line directly below the block.
+		if (inputType === 'insertLineBreak' || inputType === 'insertParagraph') {
+			const caret = rangeToTextOffset(rootElement, safeRange());
+			if (stripBlockBoundaryLineBreaks(rootElement)) {
+				restoreCaret(caret);
+			} else {
+				const source = serializeContent(rootElement);
+				let end = caret;
+
+				// the caret must end up after the inserted \n; some browsers
+				// leave it before (stuck at the end of the old line). A
+				// preceding \n means it already sits past the break
+				// (Chromium's artificial trailing newline) - leave it.
+				if (source[end] === '\n' && source[end - 1] !== '\n') {
+					end += 1;
+					restoreCaret(end);
+				}
+
+				// a line break at the buffer end renders only with a second,
+				// artificial trailing \n: a lone trailing \n is collapsed, so
+				// the new line is invisible and the next typed character
+				// consumes it. Append it when missing - unless the trailing
+				// \n doubles as a block's separator line (source ends with
+				// \n\n) or sits inside a block element.
+				let last = rootElement.lastChild;
+				while (last && last.nodeName === 'BR') last = last.previousSibling;
+				if (
+					end === source.length &&
+					source.endsWith('\n') &&
+					source[source.length - 2] !== '\n' &&
+					last?.nodeType === Node.TEXT_NODE
+				) {
+					// eslint-disable-next-line svelte/no-dom-manipulating -- the token layer is owned imperatively; Svelte renders only the contenteditable host, never its children
+					rootElement.appendChild(document.createTextNode('\n'));
+					restoreCaret(source.length);
+					resizeHeight();
+				}
+			}
+		}
+
+		syncCodeBlockHatches(rootElement);
+
+		const serialized = serializeContent(rootElement);
+		syncEmptyState(serialized);
+		if (serialized === lastEmittedValue) return;
+
+		// Plain typing/deletes coalesce per time window; structural edits
+		// (paste, newline, cut, autocorrect) start a new undo group.
+		recordHistory(inputType !== 'insertText' && !inputType?.startsWith('deleteContent'));
+
+		lastEmittedValue = serialized;
+		value = serialized;
+
+		// Rebuild when token boundaries shifted (a code span was just
+		// completed or broken) - the browser-owned text nodes cannot
+		// restyle themselves across element boundaries.
+		const tokens = tokenizeContent(serialized);
+		if (!domMatchesTokens(rootElement, tokens)) {
+			renderTokens(tokens);
+
+			// The rebuild can re-shape the DOM in a way that changes the
+			// serialization (e.g. Chromium merged trailing text into the
+			// block element and the rebuild splits it back out, which
+			// synthesizes the separator newline) - keep value in sync.
+			const reserialized = serializeContent(rootElement);
+			if (reserialized !== serialized) {
+				lastEmittedValue = reserialized;
+				value = reserialized;
+			}
+		} else {
+			rehighlightCaretCodeBlock();
+		}
+
+		onInput?.();
+	}
+
+	function handleInput(event: Event) {
+		processInput((event as InputEvent).inputType);
+	}
+
+	function handleCompositionStart() {
+		isComposing = true;
+	}
+
+	function handleCompositionEnd() {
+		isComposing = false;
+		processInput();
+	}
+
+	/**
+	 * Insert a line break at the caret MANUALLY. Native Shift+Enter at
+	 * the buffer end varies across browsers (a lone trailing \n that the
+	 * renderer collapses, or a <br> that the hatch sync strips), which
+	 * can leave the caret stuck on the old line; splitting the text node
+	 * ourselves keeps the DOM shape - and the caret - deterministic.
+	 * `processInput` then appends the artificial trailing \n when the
+	 * break lands at the buffer end.
+	 */
+	function insertLineBreak() {
+		if (!rootElement) return;
+
+		const range = safeRange();
+		if (!range) return;
+
+		if (!range.collapsed) {
+			range.deleteContents();
+		}
+
+		const container = range.startContainer;
+		const offset = range.startOffset;
+		const nl = document.createTextNode('\n');
+
+		// a break at the very end of a code block exits the block (the
+		// new line belongs below it, not inside)
+		let exitBlock: HTMLElement | null = null;
+		if (container.nodeType === Node.TEXT_NODE) {
+			let node: Node | null = container.parentNode;
+			while (node && node !== rootElement) {
+				if (node instanceof HTMLElement && node.dataset.codeToken === 'block') {
+					const tail = document.createRange();
+					tail.setStart(container, offset);
+					tail.setEnd(node, node.childNodes.length);
+					if (tail.toString().length === 0) exitBlock = node;
+					break;
+				}
+				node = node.parentNode;
+			}
+		}
+
+		if (exitBlock) {
+			exitBlock.after(nl);
+		} else if (container.nodeType === Node.TEXT_NODE) {
+			const text = container as Text;
+			if (offset === 0) {
+				text.before(nl);
+			} else if (offset === text.length) {
+				text.after(nl);
+			} else {
+				text.splitText(offset).before(nl);
+			}
+		} else {
+			container.insertBefore(nl, container.childNodes[offset] ?? null);
+		}
+
+		const selection = window.getSelection();
+		const after = document.createRange();
+		after.setStartAfter(nl);
+		after.collapse(true);
+		selection?.removeAllRanges();
+		selection?.addRange(after);
+
+		processInput('insertLineBreak');
+	}
+
+	/**
+	 * Arrow escape to the line BEFORE a leading code block. Native
+	 * caret movement has no position above a buffer-starting block,
+	 * so a transient `<br>` hatch is created on demand: it gives the
+	 * caret a visible line, is consumed by the first character typed
+	 * on it, and is removed again when the caret leaves (see
+	 * handleSelectionChange). Returns true when the caret was moved.
+	 */
+	function moveCaretBeforeLeadingCodeBlock(key: string, extend: boolean): boolean {
+		if (!rootElement) return false;
+
+		// a hatch already exists - native movement handles it
+		if (rootElement.firstChild?.nodeName === 'BR') return false;
+
+		const first = rootElement.firstChild;
+		if (!(first instanceof HTMLElement) || first.dataset.codeToken !== 'block') return false;
+
+		const range = safeRange();
+		if (!range || !range.collapsed) return false;
+
+		// the caret must sit inside the block: on its very first
+		// character for ArrowLeft, anywhere on its first line for
+		// ArrowUp
+		if (!first.contains(range.startContainer)) return false;
+
+		const caret = rangeToTextOffset(rootElement, range);
+		if (key === 'ArrowLeft') {
+			if (caret !== 0) return false;
+		} else {
+			const firstLineEnd = (first.textContent ?? '').indexOf('\n');
+			if (firstLineEnd !== -1 && caret > firstLineEnd) return false;
+		}
+
+		// eslint-disable-next-line svelte/no-dom-manipulating -- the token layer is owned imperatively; Svelte renders only the contenteditable host, never its children
+		rootElement.prepend(document.createElement('br'));
+		restoreCaret(0, extend);
+		return true;
+	}
+
+	/**
+	 * Remove the transient leading hatch once the caret leaves it.
+	 * The hatch only exists to give the caret a line above a leading
+	 * code block; with the caret anywhere else the empty line would
+	 * just be visual noise. Typing on the hatch line consumes it via
+	 * the stale-hatch removal in `syncCodeBlockHatches` instead (the
+	 * new text node takes its place before the block).
+	 */
+	function handleSelectionChange() {
+		if (!rootElement) return;
+
+		const first = rootElement.firstChild;
+		if (first?.nodeName !== 'BR') return;
+
+		const second = first.nextSibling;
+		if (!(second instanceof HTMLElement) || second.dataset.codeToken !== 'block') return;
+
+		const range = safeRange();
+		const onHatch =
+			range !== null && range.startContainer === rootElement && range.startOffset === 0;
+		if (!onHatch) {
+			first.remove();
+		}
+	}
+
+	/**
+	 * Undo/redo is replayed from source snapshots (the token rebuilds
+	 * destroy the native undo stack). Arrow keys around badges are
+	 * repaired locally: a badge is a non-editable island, so plain
+	 * ArrowLeft after a leading badge has no native previous position
+	 * and word jumps overshoot it by a word.
+	 *
+	 * Plain Enter inside a fenced code block (closed, or still open
+	 * while being typed) acts as Shift+Enter and adds a line instead of
+	 * submitting. ArrowLeft/ArrowUp at the edge of a leading code block
+	 * create the transient before-block hatch.
+	 */
+	function handleKeydown(event: KeyboardEvent) {
+		const mod = event.ctrlKey || event.metaKey;
+		if (mod && !event.altKey && !isComposing && rootElement) {
+			const key = event.key.toLowerCase();
+			const isUndo = key === 'z' && !event.shiftKey;
+			const isRedo = key === 'y' || (key === 'z' && event.shiftKey);
+
+			if (isUndo || isRedo) {
+				event.preventDefault();
+				const current = {
+					value: lastEmittedValue,
+					caret: rangeToTextOffset(rootElement, safeRange())
+				};
+				const entry = isUndo ? history.undo(current) : history.redo(current);
+				if (entry) applyHistoryEntry(entry);
+				return;
+			}
+		}
+
+		if (
+			event.key === 'Enter' &&
+			event.shiftKey &&
+			!event.ctrlKey &&
+			!event.metaKey &&
+			!event.altKey &&
+			!isIMEComposing(event) &&
+			!disabled &&
+			!caretInCodeBlock() &&
+			safeRange()
+		) {
+			// Own the break outside code blocks: native end-of-buffer
+			// behavior varies across browsers and can leave the caret
+			// stuck on the old line (see insertLineBreak).
+			event.preventDefault();
+			insertLineBreak();
+			return;
+		}
+
+		if (
+			event.key === 'Enter' &&
+			!event.shiftKey &&
+			!event.ctrlKey &&
+			!event.metaKey &&
+			!event.altKey &&
+			!isIMEComposing(event) &&
+			caretInCodeBlock()
+		) {
+			// The native plain-Enter path must never run: it splits the
+			// buffer into `<div>` wrappers that `serializeContent` cannot
+			// see. `insertLineBreak` reproduces the Shift+Enter DOM (a `\n`
+			// text node) and fires `input` synchronously, so the usual
+			// re-tokenize/re-highlight follows.
+			event.preventDefault();
+			document.execCommand('insertLineBreak');
+			return;
+		}
+
+		if (
+			rootElement &&
+			(event.key === 'ArrowLeft' || event.key === 'ArrowUp') &&
+			!event.altKey &&
+			!event.ctrlKey &&
+			!event.metaKey
+		) {
+			if (moveCaretBeforeLeadingCodeBlock(event.key, event.shiftKey)) {
+				event.preventDefault();
+				return;
+			}
+		}
+
+		if (rootElement && (event.key === 'ArrowLeft' || event.key === 'ArrowRight')) {
+			const isWordJump = (event.altKey || event.ctrlKey) && !event.metaKey;
+			const isPlainLeft =
+				event.key === 'ArrowLeft' && !event.altKey && !event.ctrlKey && !event.metaKey;
+
+			if (isWordJump || isPlainLeft) {
+				const source = serializeContent(rootElement);
+				const caret = rangeToTextOffset(rootElement, safeRange());
+				const target = isWordJump
+					? badgeAwareWordJump(source, caret, event.key === 'ArrowRight' ? 'forward' : 'backward')
+					: leadingBadgeEdgeOffset(source, caret);
+
+				if (target !== null) {
+					event.preventDefault();
+					restoreCaret(target, event.shiftKey);
+					return;
+				}
+			}
+		}
+
+		onKeydown?.(event);
+	}
+
+	// lastEmittedValue is set before `value` so the sync effect treats the
+	// change as our own and does not re-render.
+	function applyHistoryEntry(entry: SourceHistoryEntry) {
+		if (!rootElement) return;
+		renderTokens(tokenizeContent(entry.value));
+		lastEmittedValue = entry.value;
+		value = entry.value;
+		onInput?.();
+		restoreCaret(entry.caret);
+	}
+
+	/**
+	 * Plain-text paste. preventDefault + manual insertText keeps the
+	 * browser from producing stray `<div>` wrappers mid-paste; insertText
+	 * fires `input` synchronously, so `processInput` re-tokenizes the
+	 * buffer and rebuilds when the pasted text carries badge or code
+	 * tokens.
+	 */
+	function handlePasteEvent(event: ClipboardEvent) {
+		const pasted = event.clipboardData?.getData('text/plain');
+		if (pasted && pasted.length > 0) {
+			event.preventDefault();
+
+			// Snap a collapsed caret through the offset mapping first: at
+			// element-boundary carets (e.g. right before a badge) Chromium's
+			// insertText can drop the preceding text node's trailing whitespace.
+			const range = safeRange();
+			if (rootElement && range && range.collapsed) {
+				restoreCaret(rangeToTextOffset(rootElement, range));
+			}
+
+			document.execCommand('insertText', false, pasted);
+		}
+	}
+
+	// The parent's paste handler runs first and preventDefaults when it
+	// consumes the event (files, quoted prompts, long text).
+	function handlePaste(event: ClipboardEvent) {
+		onPaste?.(event);
+		if (!event.defaultPrevented) {
+			handlePasteEvent(event);
+		}
+	}
+
+	// The selection as markdown SOURCE (each badge contributes its full
+	// `[name](file://...)` link), so copy/cut carry raw markdown and
+	// pasting back re-renders the badges. Null for collapsed/outside
+	// selections - native clipboard behavior is fine there.
+	function selectionSourceSlice(): { text: string; range: Range } | null {
+		if (!rootElement) return null;
+
+		const range = safeRange();
+		if (!range || range.collapsed) return null;
+
+		const startRange = range.cloneRange();
+		startRange.collapse(true);
+
+		const source = serializeContent(rootElement);
+		const start = rangeToTextOffset(rootElement, startRange);
+		const end = rangeToTextOffset(rootElement, range);
+
+		return { text: source.slice(start, end), range };
+	}
+
+	function handleCopy(event: ClipboardEvent) {
+		const slice = selectionSourceSlice();
+		if (!slice) return;
+
+		event.clipboardData?.setData('text/plain', slice.text);
+		event.preventDefault();
+	}
+
+	function handleCut(event: ClipboardEvent) {
+		const slice = selectionSourceSlice();
+		if (!slice) return;
+
+		event.clipboardData?.setData('text/plain', slice.text);
+		event.preventDefault();
+
+		// preventDefault suppresses the native deletion, so remove the
+		// selection manually and re-emit.
+		slice.range.deleteContents();
+		processInput('deleteByCut');
+	}
+
+	onMount(() => {
+		// untrack: the DOM is managed manually from input events, so the
+		// initial render must not subscribe to the value.
+		renderTokens(tokenizeContent(untrack(() => value)));
+		lastEmittedValue = untrack(() => value ?? '');
+		resizeHeight();
+		syncEmptyState();
+		document.addEventListener('selectionchange', handleSelectionChange);
+		if (!isMobile.current) {
+			rootElement?.focus({ preventScroll: true });
+		}
+	});
+
+	onDestroy(() => {
+		document.removeEventListener('selectionchange', handleSelectionChange);
+	});
+
+	// External `value` updates. When incoming === lastEmittedValue the
+	// change came from our own input, so leave the DOM alone - the
+	// browser already owns the right shape.
+	$effect(() => {
+		const incoming = value ?? '';
+		if (incoming === lastEmittedValue) return;
+
+		recordHistory(true); // external edit (mention insert, clear, ...): own undo step
+		renderTokens(tokenizeContent(incoming));
+		lastEmittedValue = incoming;
+	});
+
+	export function getElement() {
+		return rootElement;
+	}
+
+	export function getCaretOffset(): number {
+		if (!rootElement) return 0;
+		return rangeToTextOffset(rootElement, safeRange());
+	}
+
+	// Focus first: `selection.addRange` requires it on some browsers.
+	export function setCaretOffset(offset: number) {
+		if (rootElement && rootElement !== document.activeElement) {
+			rootElement.focus({ preventScroll: true });
+		}
+		restoreCaret(offset);
+	}
+
+	export function focus() {
+		if (isMobile.current) return;
+		rootElement?.focus({ preventScroll: true });
+	}
+
+	export function resetHeight() {
+		if (rootElement) {
+			rootElement.style.height = '';
+			resizeHeight();
+		}
+	}
+</script>
+
+<div class="flex-1 {className}">
+	<div
+		bind:this={rootElement}
+		contenteditable={!disabled}
+		role="textbox"
+		aria-multiline="true"
+		aria-disabled={disabled}
+		aria-placeholder={placeholder}
+		data-placeholder={placeholder}
+		tabindex={disabled ? -1 : 0}
+		class={[
+			'chat-form-contenteditable text-md min-h-12 w-full whitespace-pre-wrap wrap-break-word border-0 bg-transparent p-0 leading-6 outline-none focus-visible:ring-0 focus-visible:ring-offset-0',
+			disabled && 'cursor-not-allowed'
+		]}
+		style="max-height: var(--max-message-height);"
+		oncompositionstart={handleCompositionStart}
+		oncompositionend={handleCompositionEnd}
+		oninput={handleInput}
+		onkeydown={handleKeydown}
+		onpaste={handlePaste}
+		oncopy={handleCopy}
+		oncut={handleCut}
+	></div>
+</div>
+
+<style>
+	/* pre-wrap is load-bearing: without it Chromium collapses \n in
+	   text nodes and converts them to spaces while typing */
+	.chat-form-contenteditable {
+		white-space: pre-wrap;
+	}
+
+	.chat-form-contenteditable:global([data-empty='true'])::before {
+		content: attr(data-placeholder);
+		color: var(--muted-foreground);
+		pointer-events: none;
+	}
+
+	/* Inline code - mirrors markdown-content.css */
+	.chat-form-contenteditable :global(code[data-code-token='inline']) {
+		background: var(--muted);
+		color: var(--muted-foreground);
+		padding: 0.125rem 0.375rem;
+		border-radius: 0.375rem;
+		font-size: 0.875rem;
+	}
+
+	/* Fenced code block - mirrors .code-block-wrapper in markdown-content.css */
+	.chat-form-contenteditable :global(code[data-code-token='block']) {
+		display: block;
+		margin: 0.25rem 0;
+		padding: 0.75rem 1rem;
+		border: 1px solid color-mix(in oklch, var(--border) 30%, transparent);
+		border-radius: 0.75rem;
+		background: var(--code-background);
+		color: var(--code-foreground);
+		font-size: 0.875rem;
+		line-height: 1.3;
+	}
+</style>

tools/ui/src/lib/components/app/chat/ChatForm/ChatFormPickers/ChatFormMentionPicker.svelte

@@ -2,7 +2,7 @@
 	import { File, Folder } from '@lucide/svelte';
 	import { abbreviateHome, runGlobSearchWithChildren, type GlobEntryResult } from '$lib/utils';
 	import { toolsStore } from '$lib/stores/tools.svelte';
-	import { BuiltInTool, FileMentionEntryType, GlobSearchType } from '$lib/enums';
+	import { BuiltInTool, FileMentionEntryType, GlobSearchType, KeyboardKey } from '$lib/enums';
 	import { isMobile } from '$lib/stores/viewport.svelte';
 	import { config } from '$lib/stores/settings.svelte';
 	import * as Popover from '$lib/components/ui/popover';
@@ -162,6 +162,17 @@
 	}
 
 	export function handleKeydown(event: KeyboardEvent): boolean {
+		// Always consume Enter while the picker is open - even with no
+		// result yet (skeletons) or no matches - so the chat form's
+		// Enter-to-submit never fires mid-search.
+		if (isOpen && event.key === KeyboardKey.ENTER) {
+			event.preventDefault();
+			if (nav.hoveredIndex >= 0 && displayedItems[nav.hoveredIndex]) {
+				handleSelect(displayedItems[nav.hoveredIndex]);
+			}
+			return true;
+		}
+
 		return nav.handleKeydown(event);
 	}
 </script>

tools/ui/src/lib/components/app/chat/ChatForm/ChatFormTextarea.svelte

@@ -48,7 +48,8 @@
 		}
 	}
 
-	// Plain-text caret offsets for the picker/paste/mention-splice flows.
+	// Plain-text caret offsets, shared with the contenteditable variant so
+	// the picker/paste flows can address either renderer through one handle.
 	export function getCaretOffset(): number {
 		if (!textareaElement) return 0;
 		return textareaElement.selectionStart ?? textareaElement.value.length;

tools/ui/src/lib/components/app/chat/index.ts

@@ -120,7 +120,8 @@ export { default as ChatAttachmentsPreviewCurrentItem } from './ChatAttachments/
  * Used by ChatScreenForm and ChatMessageEditForm for both new conversations and message editing.
  *
  * **Architecture:**
- * - Composes ChatFormTextarea, ChatFormActions, and ChatFormPickerMcpPrompts
+ * - Composes ChatFormTextarea (or ChatFormContenteditable for messages with
+ *   file mention links), ChatFormActions, and ChatFormPickerMcpPrompts
  * - Manages file upload state via `uploadedFiles` bindable prop
  * - Integrates with ModelsSelectorDropdown for model selection in router mode
  * - Communicates with parent via callbacks (onSubmit, onFilesAdd, onStop, etc.)
@@ -266,9 +267,16 @@ export { default as ChatFormFileInputInvisible } from './ChatForm/ChatFormFileIn
 export { default as ChatFormMcpResourcesList } from './ChatForm/ChatFormMcpResourcesList.svelte';
 
 /**
- * Auto-resizing textarea with IME composition support. Mention links stay
+ * Auto-resizing contenteditable input that renders `[name](file://...)`
- * plain markdown text in the input; the chip rendering happens in the
+ * mention links as inline chips while keeping the value as the markdown
- * message view via the rehype file-badge plugin.
+ * source string. ChatForm swaps it in once a mention link lands in the
+ * buffer. Shares the focus()/resetHeight()/caret handle with the textarea.
+ */
+export { default as ChatFormContenteditable } from './ChatForm/ChatFormContenteditable.svelte';
+
+/**
+ * Plain auto-resizing textarea with IME composition support. Default input
+ * renderer inside ChatForm until a file mention lands.
  */
 export { default as ChatFormTextarea } from './ChatForm/ChatFormTextarea.svelte';
 

tools/ui/src/lib/components/app/content/MarkdownContent/MarkdownContent.svelte

@@ -34,7 +34,8 @@
 		preprocessLaTeX,
 		getImageErrorFallbackHtml,
 		copyCodeToClipboard,
-		copyToClipboard
+		copyToClipboard,
+		splitGluedClosingCodeFences
 	} from '$lib/utils';
 	import {
 		IMAGE_NOT_ERROR_BOUND_SELECTOR,
@@ -342,7 +343,11 @@
 	 * Incomplete code blocks are rendered using SyntaxHighlightedCode to maintain interactivity.
 	 * @param markdown - The raw markdown string to process
 	 */
-	async function processMarkdown(markdown: string) {
+	async function processMarkdown(rawMarkdown: string) {
+		// Text glued to a closing code fence is not a fence to the parser -
+		// the block would swallow it. Split it onto its own line first.
+		const markdown = splitGluedClosingCodeFences(rawMarkdown);
+
 		// Early exit if content unchanged (can happen with rapid coalescing)
 		if (markdown === previousContent) {
 			return;

tools/ui/src/lib/components/app/content/MarkdownContent/markdown-content.css

@@ -243,7 +243,6 @@ div.markdown-user-content :global(.table-wrapper) {
 /* Code blocks */
 
 .markdown-content :global(.code-block-wrapper) {
-	margin: 1.5rem 0;
 	border-radius: 0.75rem;
 	overflow: hidden;
 	border: 1px solid color-mix(in oklch, var(--border) 30%, transparent);
@@ -253,6 +252,14 @@ div.markdown-user-content :global(.table-wrapper) {
 	max-height: var(--max-message-height);
 }
 
+.markdown-content .markdown-block:not(:first-child) :global(.code-block-wrapper) {
+	margin-top: 1rem;
+}
+
+.markdown-content .markdown-block:not(:last-child) :global(.code-block-wrapper) {
+	margin-bottom: 1rem;
+}
+
 .markdown-content:global(.dark) :global(.code-block-wrapper) {
 	border-color: color-mix(in oklch, var(--border) 20%, transparent);
 }

tools/ui/src/lib/components/app/content/MarkdownContent/plugins/rehype/file-badge.ts

@@ -1,7 +1,7 @@
 /**
  * Rehype plugin that rewrites `file://` markdown anchors into the inline
- * @-mention chip, reusing the visual contract from
+ * mention chip, sharing the class string with the contenteditable
- * `$lib/constants/mention-badge`.
+ * tokenizer via `$lib/constants/mention-badge`.
  *
  * The chip is presentational: `file://` navigation is blocked from
  * http(s) pages, so the anchor becomes a plain `<span>` (no link role,

tools/ui/src/lib/constants/css-classes.ts

@@ -19,8 +19,9 @@ export const PANEL_CLASSES = `
 export const CHAT_FORM_POPOVER_MAX_HEIGHT = 'max-h-80';
 export const DIALOG_SUBMENU_CONTENT = 'w-60';
 
-/** Selects the chat-form input to restore focus after model actions. */
+/** Selects the focused chat-form input (either renderer) to restore focus after model actions. */
-export const CHAT_INPUT_FOCUS_SELECTOR = '[data-slot="input-area"] textarea';
+export const CHAT_INPUT_FOCUS_SELECTOR =
+	'[data-slot="input-area"] textarea, [data-slot="input-area"] [contenteditable="true"]';
 
 /** Default Tailwind size class for inline icon components (lucide, etc.). */
 export const ICON_CLASS_DEFAULT = 'h-4 w-4';

tools/ui/src/lib/constants/mention-badge.ts

@@ -1,8 +1,9 @@
 /**
- * Visual contract for message @-mention badges. Svelte cannot be mounted
+ * Shared visual contract between the two DOM-only badge paths (the
- * from a hast tree, so the rehype file-badge plugin emits the shared class
+ * contenteditable tokenizer + the rehype plugin). Svelte cannot be
- * string below; keeping it here as a literal lets Tailwind's source
+ * mounted at the per-keystroke tokenizer hot path nor from a hast tree,
- * scanner generate the utility classes.
+ * so both emit the badge with the same class string literal; Tailwind's
+ * scanner picks it up in both sources.
  */
 export const MENTION_BADGE_CLASSNAME =
 	'inline-flex w-fit shrink-0 items-center gap-1 whitespace-nowrap rounded-md border border-border/50 bg-foreground/5 px-1.5 py-0.5 text-xs font-mono text-foreground hover:bg-foreground/10 dark:bg-foreground/10 dark:text-secondary-foreground';
@@ -10,8 +11,10 @@ export const MENTION_BADGE_CLASSNAME =
 export const MENTION_BADGE_ICON_CLASSNAME = 'h-3 w-3 shrink-0';
 
 /**
- * SVG attributes shared by the hast-built badge icons; the rehype plugin
+ * SVG attributes shared by the DOM-built and hast-built badge icons.
- * spreads them onto the `<svg>` `properties`.
+ * The tokenizer applies them via `setAttribute`, the rehype plugin
+ * spreads them onto the hast `<svg>` `properties`; string values are
+ * valid for both.
  */
 export const MENTION_BADGE_SVG_ATTRIBUTES: Readonly<Record<string, string>> = {
 	xmlns: 'http://www.w3.org/2000/svg',

tools/ui/src/lib/enums/keyboard.enums.ts

@@ -8,7 +8,6 @@ export enum KeyboardKey {
 	ARROW_DOWN = 'ArrowDown',
 	ARROW_LEFT = 'ArrowLeft',
 	ARROW_RIGHT = 'ArrowRight',
-	BACKSPACE = 'Backspace',
 	TAB = 'Tab',
 	B_LOWER = 'b',
 	D_LOWER = 'd',

tools/ui/src/lib/utils/code.ts

@@ -17,6 +17,56 @@ export interface IncompleteCodeBlock {
 	openingIndex: number;
 }
 
+// A fence line: up to 3 leading spaces (CommonMark), 3+ backticks, then
+// whatever trails on the same line.
+const FENCE_LINE_REGEX = /^ {0,3}(`{3,})(.*)$/;
+
+/**
+ * Splits text glued to a closing code fence onto its own line:
+ *
+ *     ```ts
+ *     let foo = 'bar';
+ *     ```create this file on ...
+ *
+ * A closing fence with trailing text is not a fence to the markdown
+ * parser, so the block would swallow the text as code. The chat form
+ * normally keeps the fence on its own line, but older messages and
+ * hand-pasted content can carry the glued form.
+ *
+ * Only trailing text containing whitespace is split: a single word
+ * after the backticks inside a fenced block is more likely nested
+ * markdown (a ```python example inside a ```md block) than glued prose.
+ */
+export function splitGluedClosingCodeFences(markdown: string): string {
+	if (!markdown.includes('```')) return markdown;
+
+	const lines = markdown.split(NEWLINE);
+	let inside = false;
+	let changed = false;
+
+	for (let i = 0; i < lines.length; i++) {
+		const match = FENCE_LINE_REGEX.exec(lines[i]);
+		if (!match) continue;
+
+		if (!inside) {
+			inside = true;
+			continue;
+		}
+
+		inside = false;
+
+		const trailing = match[2];
+		if (trailing.includes('`') || !/\s/.test(trailing)) continue;
+
+		lines[i] = lines[i].slice(0, lines[i].length - trailing.length);
+		lines.splice(i + 1, 0, trailing.trim());
+		i++;
+		changed = true;
+	}
+
+	return changed ? lines.join(NEWLINE) : markdown;
+}
+
 /**
  * Strips empty lines (whitespace-only) from the start and end of code.
  *

tools/ui/src/lib/utils/contenteditable-tokenizer.ts

@@ -0,0 +1,890 @@
+/**
+ * Maps between the chat-form contenteditable's markdown source and the
+ * badge/code/text token stream the DOM is built from. A badge is one
+ * opaque source contribution (`[name](file://path)`); its own subtree
+ * is never walked, and the caret cannot land inside it, so offsets
+ * resolve to the nearest badge edge. Code spans (`<code data-code-token>`)
+ * are EDITABLE, unlike badges: they carry the full source segment
+ * (backtick fences included) as their text, so their textContent
+ * serializes verbatim and source offsets map 1:1 to text offsets.
+ *
+ * The tokenizer emits a flat DOM (text nodes + badges + code spans),
+ * but browsers restructure it on Enter (`<div>` line wrappers, `<br>`
+ * shapes). Serialization folds those back into `\n` so the source
+ * never diverges from what is on screen; both offset mappers
+ * understand the same shapes.
+ *
+ * The newline separating a fenced block from adjacent content is a
+ * SOURCE-level concept, never stored in the DOM: the block is
+ * display:block, so a leading `\n` in the following text node would
+ * render as a phantom empty line. Serialization synthesizes exactly
+ * one `\n` at every block boundary and `buildFragment` strips it from
+ * text tokens. A text node's own leading/trailing `\n` next to a
+ * block is an ADDITIONAL blank line.
+ */
+
+import {
+	decodeFileLinkPath,
+	fileMentionLinkRe,
+	getMentionBadgeIconPaths,
+	getMentionBadgeLabel
+} from './mention-badge';
+import {
+	MENTION_BADGE_CLASSNAME,
+	MENTION_BADGE_ICON_CLASSNAME,
+	MENTION_BADGE_SVG_ATTRIBUTES,
+	SETTINGS_KEYS
+} from '$lib/constants';
+import { settingsStore } from '$lib/stores/settings.svelte';
+import { toolsStore } from '$lib/stores/tools.svelte';
+
+export type ContentToken =
+	| { kind: 'text'; text: string }
+	| { kind: 'badge'; name: string; path: string }
+	| { kind: 'inlineCode'; text: string }
+	| { kind: 'codeBlock'; text: string };
+
+// Block wrappers browsers insert for newlines; each folds back into a
+// single `\n` during serialization.
+const BLOCK_TAG_NAMES = new Set(['DIV', 'P']);
+
+// `file://` is required so plain URLs stay as text; `)` terminates only
+// when not followed by whitespace or `[` (adjacent badges keep working).
+const MENTION_BADGE_RE = fileMentionLinkRe('g');
+
+function badgeSourceLength(name: string, path: string): number {
+	if (!name || !path) return 0;
+	return `[${name}](file://${path})`.length;
+}
+
+/**
+ * Recognize complete code spans. Fenced blocks (triple backticks,
+ * optional language, possibly multiline) take priority over inline
+ * spans (single backticks, single line, non-empty). Only CLOSED
+ * spans match: an unclosed fence stays plain text until the closing
+ * backticks land. The match includes the fences so the token's
+ * source length equals its rendered text length.
+ */
+const CODE_SPAN_RE = /(```[\s\S]*?```)|(`[^`\n]+`)/g;
+
+/**
+ * Cheap gate check for `ChatForm`: does the buffer contain a
+ * complete code span (inline or fenced)? Used to promote the plain
+ * textarea to the contenteditable renderer.
+ */
+export function containsCodeSpan(value: string): boolean {
+	CODE_SPAN_RE.lastIndex = 0;
+	return CODE_SPAN_RE.test(value);
+}
+
+const CODE_FENCE_RE = /```/g;
+
+/**
+ * Is `offset` inside a fenced code block region? Toggle-based: an
+ * odd number of ``` fences before the offset means the position
+ * sits in block content. Unlike `containsCodeSpan` this also
+ * counts the still-OPEN fence while the user is typing a block
+ * (no closing ``` yet), so Enter can add a line instead of
+ * submitting the message.
+ */
+export function isOffsetInCodeBlock(source: string, offset: number): boolean {
+	let inside = false;
+	CODE_FENCE_RE.lastIndex = 0;
+
+	let match: RegExpExecArray | null;
+	while ((match = CODE_FENCE_RE.exec(source)) !== null) {
+		if (match.index + match[0].length > offset) break;
+		inside = !inside;
+	}
+
+	return inside;
+}
+
+/**
+ * Tokenize a markdown source value into the segments the
+ * contenteditable will render. Code spans are carved out first
+ * (their content is literal - a `file://` link inside backticks
+ * must NOT render as a badge), then plain text and badges
+ * interleave in the remaining gaps. Any whitespace after a badge
+ * stays in a plain text token so the round trip is byte-exact.
+ */
+export function tokenizeContent(input: string): ContentToken[] {
+	const tokens: ContentToken[] = [];
+	let cursor = 0;
+	CODE_SPAN_RE.lastIndex = 0;
+
+	let match: RegExpExecArray | null;
+	while ((match = CODE_SPAN_RE.exec(input)) !== null) {
+		const start = match.index;
+
+		if (start > cursor) {
+			pushTextAndBadgeTokens(input.slice(cursor, start), tokens);
+		}
+
+		tokens.push(
+			match[1] !== undefined
+				? { kind: 'codeBlock', text: match[1] }
+				: { kind: 'inlineCode', text: match[2] }
+		);
+		cursor = start + match[0].length;
+	}
+
+	if (cursor < input.length) {
+		pushTextAndBadgeTokens(input.slice(cursor), tokens);
+	}
+
+	return tokens;
+}
+
+/**
+ * Tokenize a code-free segment into text and badge tokens.
+ */
+function pushTextAndBadgeTokens(input: string, tokens: ContentToken[]) {
+	let cursor = 0;
+	MENTION_BADGE_RE.lastIndex = 0;
+
+	let match: RegExpExecArray | null;
+	while ((match = MENTION_BADGE_RE.exec(input)) !== null) {
+		const [whole, name, path] = match;
+		const start = match.index;
+
+		if (start > cursor) {
+			tokens.push({ kind: 'text', text: input.slice(cursor, start) });
+		}
+
+		tokens.push({ kind: 'badge', name, path });
+		cursor = start + whole.length;
+	}
+
+	if (cursor < input.length) {
+		tokens.push({ kind: 'text', text: input.slice(cursor) });
+	}
+}
+
+function isCodeBlockElement(node: Node | null): node is HTMLElement {
+	return node instanceof HTMLElement && node.dataset.codeToken === 'block';
+}
+
+/**
+ * Serialize a contenteditable subtree back to source. `<br>` and block
+ * wrappers the browser inserted for newlines fold back into `\n` (a
+ * trailing `<br>` is the browser's caret placeholder, not a newline);
+ * any other element is transparent. Code spans serialize their
+ * textContent verbatim (fences included). One separator `\n` is
+ * synthesized at every fenced-block boundary (the DOM never stores
+ * it), and a `<br>` adjacent to a code block is an escape hatch, not
+ * a newline.
+ */
+export function serializeContent(root: HTMLElement): string {
+	let out = '';
+	let pendingBlockBoundary = false;
+
+	const walk = (parent: Node) => {
+		let first = true; // no source-contributing sibling seen yet
+
+		for (const child of Array.from(parent.childNodes)) {
+			if (child.nodeType === Node.TEXT_NODE) {
+				const text = child.textContent ?? '';
+				if (text.length > 0) {
+					if (pendingBlockBoundary) {
+						out += '\n';
+						pendingBlockBoundary = false;
+					}
+					out += text;
+					first = false;
+				}
+				continue;
+			}
+
+			if (child.nodeType !== Node.ELEMENT_NODE) continue;
+
+			const el = child as HTMLElement;
+
+			if (el.dataset.mentionBadge === 'true') {
+				const name = el.dataset.mentionName ?? '';
+				const path = el.dataset.mentionPath ?? '';
+				if (name && path) {
+					if (pendingBlockBoundary) {
+						out += '\n';
+						pendingBlockBoundary = false;
+					}
+					out += `[${name}](file://${path})`;
+					first = false;
+				}
+				continue;
+			}
+
+			if (el.dataset.codeToken !== undefined) {
+				const isBlock = el.dataset.codeToken === 'block';
+				if (isBlock && (pendingBlockBoundary || !first)) out += '\n';
+				pendingBlockBoundary = false;
+				walk(el);
+				first = false;
+				if (isBlock) pendingBlockBoundary = true;
+				continue;
+			}
+
+			if (el.tagName === 'BR') {
+				const isHatch =
+					isCodeBlockElement(el.previousSibling) || isCodeBlockElement(el.nextSibling);
+				if (!isHatch && el.nextSibling) {
+					if (pendingBlockBoundary) {
+						out += '\n';
+						pendingBlockBoundary = false;
+					}
+					out += '\n';
+					first = false;
+				}
+				continue;
+			}
+
+			if (BLOCK_TAG_NAMES.has(el.tagName)) {
+				if (pendingBlockBoundary || !first) out += '\n';
+				pendingBlockBoundary = false;
+				walk(el);
+				first = false;
+				continue;
+			}
+
+			walk(el);
+			if (pendingBlockBoundary) first = false;
+		}
+	};
+
+	walk(root);
+	return out;
+}
+
+/**
+ * Compare the live DOM's non-text structure against a token stream.
+ * Only element contributions are compared (badges by name/path, code
+ * spans by kind and source segment): text nodes are owned by the
+ * browser between rebuilds, so their split/merge state is irrelevant.
+ * A mismatch means token boundaries shifted (a code span was just
+ * completed or broken) and the DOM needs a rebuild to restyle.
+ */
+export function domMatchesTokens(root: HTMLElement, tokens: ContentToken[]): boolean {
+	const expected = tokens.filter((token) => token.kind !== 'text');
+	let index = 0;
+
+	const walk = (parent: Node): boolean => {
+		for (const child of Array.from(parent.childNodes)) {
+			if (child.nodeType !== Node.ELEMENT_NODE) continue;
+
+			const el = child as HTMLElement;
+			const isBadge = el.dataset.mentionBadge === 'true';
+			const isCode = el.dataset.codeToken !== undefined;
+
+			if (!isBadge && !isCode) {
+				if (!walk(el)) return false;
+				continue;
+			}
+
+			const token = expected[index++];
+			if (!token) return false;
+
+			if (isBadge) {
+				if (token.kind !== 'badge') return false;
+				if (token.name !== (el.dataset.mentionName ?? '')) return false;
+				if (token.path !== (el.dataset.mentionPath ?? '')) return false;
+				continue;
+			}
+
+			const codeKind = el.dataset.codeToken === 'block' ? 'codeBlock' : 'inlineCode';
+			if (token.kind !== codeKind) return false;
+			if (
+				(token.kind === 'inlineCode' || token.kind === 'codeBlock') &&
+				token.text !== (el.textContent ?? '')
+			) {
+				return false;
+			}
+		}
+
+		return true;
+	};
+
+	return walk(root) && index === expected.length;
+}
+
+/**
+ * Plain-text offset of a `Range` in the root; null range (selection
+ * lost) falls back to buffer length. Walked against the live DOM (not
+ * a clone) so a `<br>` keeps its trailing/not-trailing context. Code
+ * spans count their full textContent (fences included) and the caret
+ * may land inside them; synthesized block boundaries count one `\n`
+ * once the caret is past them.
+ */
+export function rangeToTextOffset(root: HTMLElement, range: Range | null): number {
+	if (!range) return serializeContent(root).length;
+
+	// A point is at/before the caret iff it falls inside [root start, caret].
+	const pre = range.cloneRange();
+	pre.selectNodeContents(root);
+	pre.setEnd(range.endContainer, range.endOffset);
+	const atOrBeforeCaret = (node: Node, offset: number) => pre.comparePoint(node, offset) !== 1;
+
+	let total = 0;
+	let done = false;
+	// DOM position of a code block's synthesized after-boundary, set
+	// when walking past a block and consumed by the next contributing
+	// sibling (counts one `\n` once the caret is past it).
+	let pendingPoint: { node: Node; index: number } | null = null;
+
+	const walk = (parent: Node) => {
+		let first = true;
+
+		for (const child of Array.from(parent.childNodes)) {
+			if (done) return;
+
+			if (child.nodeType === Node.TEXT_NODE) {
+				const text = child.textContent ?? '';
+				if (text.length === 0) continue;
+				if (pendingPoint) {
+					const { node, index } = pendingPoint;
+					pendingPoint = null;
+					if (!atOrBeforeCaret(node, index)) {
+						done = true;
+						return;
+					}
+					total += 1;
+				}
+				if (!atOrBeforeCaret(child, 0)) {
+					done = true;
+					return;
+				}
+				if (range.endContainer === child) {
+					total += range.endOffset;
+					done = true;
+					return;
+				}
+				total += text.length;
+				first = false;
+				continue;
+			}
+
+			if (child.nodeType !== Node.ELEMENT_NODE) continue;
+
+			const el = child as HTMLElement;
+			const parentNode = el.parentNode as Node;
+			const elIndex = Array.prototype.indexOf.call(parentNode.childNodes, el);
+
+			if (pendingPoint) {
+				const { node, index } = pendingPoint;
+				pendingPoint = null;
+				if (!atOrBeforeCaret(node, index)) {
+					done = true;
+					return;
+				}
+				total += 1;
+			}
+
+			if (el.dataset.mentionBadge === 'true') {
+				const len = badgeSourceLength(el.dataset.mentionName ?? '', el.dataset.mentionPath ?? '');
+				if (len === 0) continue;
+				if (!atOrBeforeCaret(parentNode, elIndex + 1)) {
+					done = true;
+					return;
+				}
+				total += len;
+				first = false;
+				continue;
+			}
+
+			if (el.dataset.codeToken !== undefined) {
+				const isBlock = el.dataset.codeToken === 'block';
+				if (isBlock && !first) {
+					if (!atOrBeforeCaret(el, 0)) {
+						done = true;
+						return;
+					}
+					total += 1;
+				}
+				walk(el);
+				first = false;
+				if (isBlock) pendingPoint = { node: parentNode, index: elIndex + 1 };
+				continue;
+			}
+
+			if (el.tagName === 'BR') {
+				const isHatch =
+					isCodeBlockElement(el.previousSibling) || isCodeBlockElement(el.nextSibling);
+				if (isHatch || !el.nextSibling) continue;
+				if (!atOrBeforeCaret(parentNode, elIndex + 1)) {
+					done = true;
+					return;
+				}
+				total += 1;
+				first = false;
+				continue;
+			}
+
+			if (BLOCK_TAG_NAMES.has(el.tagName)) {
+				if (!first) {
+					if (!atOrBeforeCaret(el, 0)) {
+						done = true;
+						return;
+					}
+					total += 1;
+				}
+				walk(el);
+				first = false;
+				continue;
+			}
+
+			const before = total;
+			walk(el);
+			if (total > before) first = false;
+		}
+	};
+
+	walk(root);
+	return total;
+}
+
+/**
+ * Materialize a token stream into a DOM subtree: text nodes for text
+ * tokens, `<span data-mention-badge="true">` elements for badges,
+ * `<code data-code-token>` elements for code spans. The badge's class
+ * string + inline SVG are shared with the rehype plugin via
+ * `$lib/constants/mention-badge`.
+ */
+export function buildFragment(tokens: ContentToken[]): DocumentFragment {
+	const fragment = document.createDocumentFragment();
+
+	for (let index = 0; index < tokens.length; index++) {
+		const token = tokens[index];
+
+		if (token.kind === 'text') {
+			let text = token.text;
+
+			// The separator \n at a fenced-block boundary is synthesized
+			// at serialization time; keeping it in the DOM would render a
+			// phantom empty line next to the block.
+			if (tokens[index - 1]?.kind === 'codeBlock' && text.startsWith('\n')) {
+				text = text.slice(1);
+			}
+			if (tokens[index + 1]?.kind === 'codeBlock' && text.endsWith('\n')) {
+				text = text.slice(0, -1);
+			}
+			if (text.length === 0) continue;
+
+			fragment.appendChild(document.createTextNode(text));
+			continue;
+		}
+
+		if (token.kind === 'inlineCode' || token.kind === 'codeBlock') {
+			const code = document.createElement('code');
+			code.dataset.codeToken = token.kind === 'codeBlock' ? 'block' : 'inline';
+			code.textContent = token.text;
+			fragment.appendChild(code);
+			continue;
+		}
+
+		// A leading badge gets an empty text node prepended: without a real
+		// text position at the buffer start, the spot before the badge is
+		// unreachable via keyboard (ArrowLeft/Home).
+		if (!fragment.lastChild) {
+			fragment.appendChild(document.createTextNode(''));
+		}
+
+		const badge = document.createElement('span');
+		badge.dataset.mentionBadge = 'true';
+		badge.dataset.mentionName = token.name;
+		badge.dataset.mentionPath = token.path;
+		badge.title = decodeFileLinkPath(token.path);
+		badge.className = MENTION_BADGE_CLASSNAME;
+		badge.contentEditable = 'false';
+
+		const svg = document.createElementNS(MENTION_BADGE_SVG_ATTRIBUTES['xmlns'], 'svg');
+		for (const [attr, value] of Object.entries(MENTION_BADGE_SVG_ATTRIBUTES)) {
+			svg.setAttribute(attr, value);
+		}
+		for (const cls of MENTION_BADGE_ICON_CLASSNAME.split(/\s+/).filter(Boolean)) {
+			svg.classList.add(cls);
+		}
+
+		for (const d of getMentionBadgeIconPaths(token.path)) {
+			const path = document.createElementNS(MENTION_BADGE_SVG_ATTRIBUTES['xmlns'], 'path');
+			path.setAttribute('d', d);
+			svg.appendChild(path);
+		}
+
+		const label = document.createElement('span');
+		label.classList.add('shrink-0', 'truncate');
+		label.textContent = getMentionBadgeLabel(
+			token.name,
+			decodeFileLinkPath(token.path),
+			settingsStore.getConfig(SETTINGS_KEYS.SHOW_FULL_PATH_IN_MENTIONS),
+			toolsStore.serverHome
+		);
+
+		badge.appendChild(svg);
+		badge.appendChild(label);
+		fragment.appendChild(badge);
+	}
+
+	return fragment;
+}
+
+// A sibling provides a reachable caret line when it is an element
+// (badge, another block, an existing hatch) or a non-empty text node.
+function hasLineBeside(node: Node | null): boolean {
+	if (!node) return false;
+	if (node.nodeType === Node.ELEMENT_NODE) return true;
+	return (node.textContent ?? '') !== '';
+}
+
+/**
+ * A code block at the END of the buffer needs an editable line after
+ * it: without one the caret cannot leave the block with
+ * ArrowDown/ArrowRight. A trailing `<br>` provides that line while
+ * staying transparent to serialization (skipped as a hatch), and is
+ * removed again once real content takes its place.
+ *
+ * No hatch is added BEFORE a leading block: the empty line above it
+ * is transient and managed by the component (created when the caret
+ * arrows onto it, removed when the caret leaves). A transient
+ * leading hatch found here is kept; the browser's lone placeholder
+ * `<br>` in an empty root is left untouched.
+ */
+export function syncCodeBlockHatches(root: HTMLElement) {
+	for (const child of Array.from(root.childNodes)) {
+		if (child.nodeName !== 'BR') continue;
+
+		const isPlaceholder = root.childNodes.length === 1;
+		const isLeadingHatch = !child.previousSibling && isCodeBlockElement(child.nextSibling);
+		const isTrailingHatch = !child.nextSibling && isCodeBlockElement(child.previousSibling);
+
+		// A hatch goes stale once real content takes over its line:
+		// content before a leading hatch, content after a trailing one,
+		// or a text node after the block already providing the line.
+		// A `<br>` with no code block around is a real newline (browser
+		// Shift+Enter shape) and stays.
+		let prevElement = child.previousSibling;
+		while (prevElement && prevElement.nodeType !== Node.ELEMENT_NODE) {
+			prevElement = prevElement.previousSibling;
+		}
+		const nearBlock =
+			isCodeBlockElement(child.nextSibling) ||
+			isCodeBlockElement(child.previousSibling) ||
+			isCodeBlockElement(prevElement);
+
+		if (!isPlaceholder && !isLeadingHatch && !isTrailingHatch && nearBlock) {
+			child.remove();
+		}
+	}
+
+	for (const child of Array.from(root.childNodes)) {
+		if (!isCodeBlockElement(child)) continue;
+
+		if (!hasLineBeside(child.nextSibling)) {
+			child.after(document.createElement('br'));
+		}
+	}
+}
+
+/**
+ * Strip the separator and artificial newlines from an all-newline text
+ * node directly after a fenced block. Chromium's line break at the
+ * buffer end inserts an extra artificial `\n` so the new line has
+ * height, and the first `\n` after a block doubles as the fence's
+ * separator line (synthesized at serialization time). Removing both
+ * makes Shift+Enter after a block land the caret on the line directly
+ * below the block, like a plain textarea would.
+ *
+ * Only all-newline text nodes are touched: a node with real content
+ * carries intentional blank lines and is left alone. Returns true when
+ * the DOM changed.
+ */
+export function stripBlockBoundaryLineBreaks(root: HTMLElement): boolean {
+	let changed = false;
+
+	for (const child of Array.from(root.childNodes)) {
+		if (child.nodeType !== Node.TEXT_NODE) continue;
+		if (!isCodeBlockElement(child.previousSibling)) continue;
+
+		let text = child.textContent ?? '';
+		if (!/^\n{2,}$/.test(text)) continue;
+
+		text = text.slice(1);
+
+		const atBufferEnd = !child.nextSibling || child.nextSibling.nodeName === 'BR';
+		if (atBufferEnd) {
+			text = text.slice(0, -1);
+		}
+
+		child.textContent = text;
+		changed = true;
+	}
+
+	return changed;
+}
+
+const WORD_CHAR_RE = /[\p{L}\p{N}_]/u;
+
+/**
+ * Word-jump target (Option+Arrow / Ctrl+Arrow) in source offsets, or null
+ * when the jump crosses no badge and native word movement should handle
+ * it. Badge spans are masked to word characters, so a badge counts as
+ * exactly one word.
+ */
+export function badgeAwareWordJump(
+	source: string,
+	offset: number,
+	direction: 'forward' | 'backward'
+): number | null {
+	let masked = '';
+	const badgeSpans: Array<[number, number]> = [];
+
+	for (const token of tokenizeContent(source)) {
+		const len =
+			token.kind === 'badge' ? badgeSourceLength(token.name, token.path) : token.text.length;
+		if (token.kind === 'badge') badgeSpans.push([masked.length, masked.length + len]);
+		masked += token.kind === 'badge' ? 'a'.repeat(len) : token.text;
+	}
+
+	if (badgeSpans.length === 0) return null;
+
+	const isWord = (index: number) => WORD_CHAR_RE.test(masked[index]);
+	const spanStartingAt = (index: number) => badgeSpans.find(([start]) => start === index);
+	const spanEndingAt = (index: number) => badgeSpans.find(([, end]) => end === index);
+	const n = masked.length;
+	let i = offset;
+
+	if (direction === 'forward') {
+		// Entering a badge completes the word phase at the badge's end edge.
+		if (!(i < n && isWord(i))) {
+			while (i < n && !isWord(i)) i++;
+		}
+		while (i < n && isWord(i)) {
+			const span = spanStartingAt(i);
+			if (span) {
+				i = span[1];
+				break;
+			}
+			i++;
+		}
+	} else {
+		if (!(i > 0 && isWord(i - 1))) {
+			while (i > 0 && !isWord(i - 1)) i--;
+		}
+		while (i > 0 && isWord(i - 1)) {
+			const span = spanEndingAt(i);
+			if (span) {
+				i = span[0];
+				break;
+			}
+			i--;
+		}
+	}
+
+	if (i === offset) return null;
+
+	const lo = Math.min(offset, i);
+	const hi = Math.max(offset, i);
+	return badgeSpans.some(([start, end]) => start < hi && end > lo) ? i : null;
+}
+
+/**
+ * 0 when `caret` sits exactly at a leading badge's end edge, null
+ * otherwise. Plain ArrowLeft there has no native previous position, so
+ * the host snaps the caret to the buffer start manually.
+ */
+export function leadingBadgeEdgeOffset(source: string, caret: number): number | null {
+	const [first] = tokenizeContent(source);
+	if (!first || first.kind !== 'badge') return null;
+	return caret === badgeSourceLength(first.name, first.path) ? 0 : null;
+}
+
+/**
+ * Translate a plain-text offset into a degenerate `Range` at that
+ * position in the DOM; out-of-range offsets clamp to buffer end (before
+ * a trailing escape hatch, not after it). Zero offset lands BEFORE a
+ * badge or code span, and an offset exactly at a code span's end lands
+ * AFTER it, so typing at a code span's edge extends the surrounding
+ * text. Interior code-span offsets land in the element's text.
+ * Understands the same block/`<br>` newline shapes as
+ * `serializeContent`.
+ */
+export function textOffsetToRange(root: HTMLElement, offset: number): Range {
+	const range = document.createRange();
+	let remaining = offset;
+	let landed = false;
+	let pendingBlockBoundary = false;
+
+	const land = (node: Node, nodeOffset: number) => {
+		range.setStart(node, nodeOffset);
+		range.setEnd(node, nodeOffset);
+		landed = true;
+	};
+
+	const walk = (parent: Node) => {
+		let first = true;
+
+		for (const child of Array.from(parent.childNodes)) {
+			if (landed) return;
+
+			if (child.nodeType === Node.TEXT_NODE) {
+				const text = child.textContent ?? '';
+				if (text.length === 0) continue;
+				if (pendingBlockBoundary) {
+					// The synthesized separator maps to the near edge of the
+					// content that follows the block.
+					pendingBlockBoundary = false;
+					if (remaining === 0) {
+						land(child, 0);
+						return;
+					}
+					remaining -= 1;
+				}
+				if (remaining <= text.length) {
+					land(child, remaining);
+					return;
+				}
+				remaining -= text.length;
+				first = false;
+				continue;
+			}
+
+			if (child.nodeType !== Node.ELEMENT_NODE) continue;
+
+			const el = child as HTMLElement;
+
+			if (el.dataset.mentionBadge === 'true') {
+				const len = badgeSourceLength(el.dataset.mentionName ?? '', el.dataset.mentionPath ?? '');
+				if (len === 0) continue;
+				if (pendingBlockBoundary) {
+					pendingBlockBoundary = false;
+					if (remaining === 0) {
+						range.setStartBefore(el);
+						range.setEndBefore(el);
+						landed = true;
+						return;
+					}
+					remaining -= 1;
+				}
+				if (remaining <= len) {
+					if (remaining === 0) {
+						range.setStartBefore(el);
+						range.setEndBefore(el);
+					} else {
+						range.setStartAfter(el);
+						range.setEndAfter(el);
+					}
+					landed = true;
+					return;
+				}
+				remaining -= len;
+				first = false;
+				continue;
+			}
+
+			if (el.dataset.codeToken !== undefined) {
+				const isBlock = el.dataset.codeToken === 'block';
+				if (isBlock && (pendingBlockBoundary || !first)) {
+					pendingBlockBoundary = false;
+					if (remaining === 0) {
+						range.setStartBefore(el);
+						range.setEndBefore(el);
+						landed = true;
+						return;
+					}
+					remaining -= 1;
+				}
+
+				const len = (el.textContent ?? '').length;
+				if (remaining === 0) {
+					range.setStartBefore(el);
+					range.setEndBefore(el);
+					landed = true;
+					return;
+				}
+				if (remaining === len) {
+					range.setStartAfter(el);
+					range.setEndAfter(el);
+					landed = true;
+					return;
+				}
+				if (remaining < len) {
+					walk(el);
+					return;
+				}
+				remaining -= len;
+				if (isBlock) remaining -= 1;
+				first = false;
+				continue;
+			}
+
+			if (el.tagName === 'BR') {
+				const isHatch =
+					isCodeBlockElement(el.previousSibling) || isCodeBlockElement(el.nextSibling);
+				if (isHatch) {
+					// Escape hatch: no source length; offset 0 lands before it
+					// so text typed there takes its place.
+					if (remaining === 0) {
+						range.setStartBefore(el);
+						range.setEndBefore(el);
+						landed = true;
+					}
+					continue;
+				}
+				if (!el.nextSibling) continue;
+				if (pendingBlockBoundary) {
+					pendingBlockBoundary = false;
+					if (remaining === 0) {
+						range.setStartBefore(el);
+						range.setEndBefore(el);
+						landed = true;
+						return;
+					}
+					remaining -= 1;
+				}
+				if (remaining === 0) {
+					range.setStartBefore(el);
+					range.setEndBefore(el);
+					landed = true;
+					return;
+				}
+				remaining -= 1;
+				first = false;
+				continue;
+			}
+
+			if (BLOCK_TAG_NAMES.has(el.tagName)) {
+				if (pendingBlockBoundary || !first) {
+					pendingBlockBoundary = false;
+					if (remaining === 0) {
+						// The boundary newline belongs to the previous line.
+						range.setStartBefore(el);
+						range.setEndBefore(el);
+						landed = true;
+						return;
+					}
+					remaining -= 1;
+				}
+				walk(el);
+				first = false;
+				continue;
+			}
+
+			const before = remaining;
+			walk(el);
+			if (remaining < before) first = false;
+		}
+	};
+
+	walk(root);
+
+	if (!landed) {
+		const last = root.lastChild;
+		if (last && last.nodeName === 'BR') {
+			range.setStartBefore(last);
+			range.setEndBefore(last);
+		} else {
+			range.selectNodeContents(root);
+			range.collapse(false);
+		}
+	}
+
+	return range;
+}

tools/ui/src/lib/utils/index.ts

@@ -33,6 +33,7 @@ export {
 export {
 	highlightCode,
 	detectIncompleteCodeBlock,
+	splitGluedClosingCodeFences,
 	trimCodePadding,
 	type IncompleteCodeBlock
 } from './code';
@@ -205,9 +206,30 @@ export {
 	type CommandDismissSnapshot
 } from './command-token';
 
-// Mention-chip visual contract shared by the rehype file-badge plugin,
+// Tokenization for the chat-form contenteditable (mention links + code spans <-> chip DOM)
-// plus the `[name](file://...)` link helpers the mention picker splices in
 export {
+	tokenizeContent,
+	containsCodeSpan,
+	isOffsetInCodeBlock,
+	domMatchesTokens,
+	syncCodeBlockHatches,
+	stripBlockBoundaryLineBreaks,
+	serializeContent,
+	buildFragment,
+	rangeToTextOffset,
+	textOffsetToRange,
+	badgeAwareWordJump,
+	leadingBadgeEdgeOffset,
+	type ContentToken
+} from './contenteditable-tokenizer';
+
+// Source-space undo/redo history for the chat-form contenteditable
+export { SourceHistory, type SourceHistoryEntry } from './source-history';
+
+// Mention-badge visual contract (used by the contenteditable / rehype
+// DOM paths that build the same chip without a Svelte mount)
+export {
+	containsFileMentionLink,
 	fileMentionLinkRe,
 	encodeFileLinkPath,
 	decodeFileLinkPath,
@@ -218,8 +240,7 @@ export {
 	MENTION_BADGE_FOLDER_ICON_PATHS,
 	getMentionBadgeIconPaths,
 	getMentionBadgeLabel,
-	buildMentionInsertion,
+	buildMentionInsertion
-	mentionLinkEndingAt
 } from './mention-badge';
 
 // Agentic content utilities (structured section derivation)

tools/ui/src/lib/utils/mention-badge.ts

@@ -23,6 +23,10 @@ export function fileMentionLinkRe(flags = ''): RegExp {
 	return new RegExp(FILE_MENTION_LINK_SOURCE, flags);
 }
 
+export function containsFileMentionLink(value: string): boolean {
+	return fileMentionLinkRe().test(value);
+}
+
 // Escape each path segment for a markdown link destination (spaces/parens
 // break CommonMark); keeps the trailing slash that marks a directory.
 export function encodeFileLinkPath(path: string): string {
@@ -60,25 +64,6 @@ export function getMentionBadgeLabel(
 	return abbreviateHome(decoded, home);
 }
 
-/**
- * Extent of the mention link ending exactly at `caret`, so Backspace there
- * deletes the whole `[name](file://...)` token in one keystroke instead of
- * unraveling it character by character. Null when no link ends at `caret`.
- */
-export function mentionLinkEndingAt(
-	value: string,
-	caret: number
-): { start: number; end: number } | null {
-	const re = fileMentionLinkRe('g');
-	let match: RegExpExecArray | null;
-	while ((match = re.exec(value)) !== null) {
-		const end = match.index + match[0].length;
-		if (end === caret) return { start: match.index, end };
-		if (end > caret) break;
-	}
-	return null;
-}
-
 /**
  * Build the markdown link that replaces a mention token. Entry `path` is
  * already rooted, so `file://` + `/abs` yields the canonical `file:///`.

tools/ui/src/lib/utils/source-history.ts

@@ -0,0 +1,48 @@
+/**
+ * Source-space undo/redo history for the chat-form contenteditable, whose
+ * imperative DOM rebuilds destroy the browser's native undo stack.
+ * Entries record the state BEFORE an edit; edits within `groupWindowMs`
+ * extend the open group so a typing burst undoes as a unit, while
+ * structural edits (paste, mention insert, clear) pass `newGroup`.
+ */
+
+export interface SourceHistoryEntry {
+	value: string;
+	caret: number;
+}
+
+export class SourceHistory {
+	private undoStack: SourceHistoryEntry[] = [];
+	private redoStack: SourceHistoryEntry[] = [];
+	private lastPush = 0;
+
+	constructor(
+		private limit = 100,
+		private groupWindowMs = 800
+	) {}
+
+	push(entry: SourceHistoryEntry, now: number, newGroup = false): void {
+		if (newGroup || now - this.lastPush >= this.groupWindowMs || this.undoStack.length === 0) {
+			this.undoStack.push(entry);
+			if (this.undoStack.length > this.limit) this.undoStack.shift();
+		}
+		this.lastPush = now;
+		this.redoStack = [];
+	}
+
+	undo(current: SourceHistoryEntry): SourceHistoryEntry | null {
+		const entry = this.undoStack.pop();
+		if (!entry) return null;
+		this.redoStack.push(current);
+		this.lastPush = 0; // the next edit after an undo starts a new group
+		return entry;
+	}
+
+	redo(current: SourceHistoryEntry): SourceHistoryEntry | null {
+		const entry = this.redoStack.pop();
+		if (!entry) return null;
+		this.undoStack.push(current);
+		this.lastPush = 0;
+		return entry;
+	}
+}

tools/ui/tests/client/chat-form-contenteditable-blocks.svelte.test.ts

@@ -0,0 +1,156 @@
+// Guards the newline contract of the chat-form contenteditable: browsers
+// restructure the flat DOM on Enter (`<div>` wrappers, `<br>` shapes) and
+// serialization must fold those back into `\n` so the emitted value never
+// diverges from what is on screen.
+
+import { describe, it, expect } from 'vitest';
+import { render } from 'vitest-browser-svelte';
+import { tick } from 'svelte';
+import ChatFormContenteditableHarness from './components/ChatFormContenteditableHarness.svelte';
+
+const SOURCE = 'see [docs](file:///a/b) here';
+
+function editableIn(container: HTMLElement): HTMLElement {
+	const el = container.querySelector('[role="textbox"]');
+	if (!(el instanceof HTMLElement)) throw new Error('contenteditable not rendered');
+	return el;
+}
+
+function fireInput(root: HTMLElement) {
+	root.dispatchEvent(new InputEvent('input', { bubbles: true }));
+}
+
+function setCaret(node: Node, offset: number) {
+	const range = document.createRange();
+	range.setStart(node, offset);
+	range.setEnd(node, offset);
+	const selection = window.getSelection();
+	if (!selection) throw new Error('no selection');
+	selection.removeAllRanges();
+	selection.addRange(range);
+}
+
+describe('ChatFormContenteditable browser newline shapes', () => {
+	it('serializes a Chromium Enter <div> wrapper as a newline', async () => {
+		const screen = render(ChatFormContenteditableHarness, { value: SOURCE });
+		await tick();
+
+		const root = editableIn(screen.container);
+		const div = document.createElement('div');
+		div.textContent = 'second line';
+		root.appendChild(div);
+		fireInput(root);
+		await tick();
+
+		expect(screen.component.getValue()).toBe(`${SOURCE}\nsecond line`);
+	});
+
+	it('serializes a Firefox full <div> wrap as lines, badge included', async () => {
+		const screen = render(ChatFormContenteditableHarness, { value: SOURCE });
+		await tick();
+
+		const root = editableIn(screen.container);
+		const first = document.createElement('div');
+		while (root.firstChild) first.appendChild(root.firstChild);
+		const second = document.createElement('div');
+		second.textContent = 'second line';
+		root.appendChild(first);
+		root.appendChild(second);
+		fireInput(root);
+		await tick();
+
+		expect(screen.component.getValue()).toBe(`${SOURCE}\nsecond line`);
+	});
+
+	it('serializes a <br> as a newline', async () => {
+		const screen = render(ChatFormContenteditableHarness, { value: 'here' });
+		await tick();
+
+		const root = editableIn(screen.container);
+		root.appendChild(document.createElement('br'));
+		root.appendChild(document.createTextNode('second line'));
+		fireInput(root);
+		await tick();
+
+		expect(screen.component.getValue()).toBe('here\nsecond line');
+	});
+
+	it('ignores a trailing <br> (browser caret placeholder)', async () => {
+		const screen = render(ChatFormContenteditableHarness, { value: 'abc' });
+		await tick();
+
+		const root = editableIn(screen.container);
+		root.appendChild(document.createElement('br'));
+		fireInput(root);
+		await tick();
+
+		expect(screen.component.getValue()).toBe('abc');
+	});
+
+	it('serializes one newline per empty-line <div><br></div>', async () => {
+		const screen = render(ChatFormContenteditableHarness, { value: 'abc' });
+		await tick();
+
+		const root = editableIn(screen.container);
+		for (let i = 0; i < 2; i++) {
+			const div = document.createElement('div');
+			div.appendChild(document.createElement('br'));
+			root.appendChild(div);
+		}
+		fireInput(root);
+		await tick();
+
+		expect(screen.component.getValue()).toBe('abc\n\n');
+	});
+
+	it('treats a <div><br></div>-only buffer as empty for the placeholder', async () => {
+		const screen = render(ChatFormContenteditableHarness, { value: 'abc' });
+		await tick();
+
+		const root = editableIn(screen.container);
+		const div = document.createElement('div');
+		div.appendChild(document.createElement('br'));
+		root.replaceChildren(div);
+		fireInput(root);
+		await tick();
+
+		expect(screen.component.getValue()).toBe('');
+		expect(root.dataset.empty).toBe('true');
+	});
+
+	it('maps the caret across block boundaries in both directions', async () => {
+		const screen = render(ChatFormContenteditableHarness, { value: 'abc\ndef' });
+		await tick();
+
+		// Rebuild into the Chromium block shape; the source is unchanged,
+		// so no re-render fires.
+		const root = editableIn(screen.container);
+		const div = document.createElement('div');
+		div.textContent = 'def';
+		root.replaceChildren(document.createTextNode('abc'), div);
+		fireInput(root);
+		await tick();
+		expect(screen.component.getValue()).toBe('abc\ndef');
+
+		const divText = div.firstChild;
+		if (!divText) throw new Error('div text missing');
+
+		setCaret(divText, 2);
+		expect(screen.component.getCaretOffset()).toBe(6);
+
+		screen.component.setCaretOffset(6);
+		const selection = window.getSelection();
+		expect(selection?.anchorNode).toBe(divText);
+		expect(selection?.anchorOffset).toBe(2);
+
+		// The boundary newline itself: offset 3 is the end of "abc", offset
+		// 4 the start of the "def" line.
+		screen.component.setCaretOffset(4);
+		expect(window.getSelection()?.anchorNode).toBe(divText);
+		expect(window.getSelection()?.anchorOffset).toBe(0);
+
+		screen.component.setCaretOffset(3);
+		expect(window.getSelection()?.anchorNode).toBe(root.firstChild);
+		expect(window.getSelection()?.anchorOffset).toBe(3);
+	});
+});

tools/ui/tests/client/chat-form-contenteditable-undo.svelte.test.ts

@@ -0,0 +1,142 @@
+// Guards the editing-key contract of the chat-form contenteditable:
+// undo/redo is replayed from source snapshots (the token rebuilds destroy
+// the native undo stack), and Tab is NOT intercepted (WCAG 2.1.2 no
+// keyboard trap), matching the plain textarea.
+
+import { describe, it, expect } from 'vitest';
+import { render } from 'vitest-browser-svelte';
+import { tick } from 'svelte';
+import ChatFormContenteditableHarness from './components/ChatFormContenteditableHarness.svelte';
+
+const SOURCE = 'see [docs](file:///a/b)';
+
+function editableIn(container: HTMLElement): HTMLElement {
+	const el = container.querySelector('[role="textbox"]');
+	if (!(el instanceof HTMLElement)) throw new Error('contenteditable not rendered');
+	return el;
+}
+
+function type(root: HTMLElement, text: string, inputType = 'insertText') {
+	root.appendChild(document.createTextNode(text));
+	root.dispatchEvent(new InputEvent('input', { bubbles: true, inputType }));
+}
+
+function keydown(root: HTMLElement, init: KeyboardEventInit) {
+	const event = new KeyboardEvent('keydown', { bubbles: true, cancelable: true, ...init });
+	root.dispatchEvent(event);
+	return event;
+}
+
+describe('ChatFormContenteditable undo/redo', () => {
+	it('undoes and redoes an edit across a badge-containing buffer', async () => {
+		const screen = render(ChatFormContenteditableHarness, { value: SOURCE });
+		await tick();
+
+		const root = editableIn(screen.container);
+		type(root, ' more');
+		await tick();
+		expect(screen.component.getValue()).toBe(`${SOURCE} more`);
+
+		const undoEvent = keydown(root, { key: 'z', ctrlKey: true });
+		await tick();
+		expect(undoEvent.defaultPrevented).toBe(true);
+		expect(screen.component.getValue()).toBe(SOURCE);
+
+		const redoEvent = keydown(root, { key: 'z', ctrlKey: true, shiftKey: true });
+		await tick();
+		expect(redoEvent.defaultPrevented).toBe(true);
+		expect(screen.component.getValue()).toBe(`${SOURCE} more`);
+	});
+
+	it('redoes with Ctrl+Y as well', async () => {
+		const screen = render(ChatFormContenteditableHarness, { value: SOURCE });
+		await tick();
+
+		const root = editableIn(screen.container);
+		type(root, ' more');
+		await tick();
+		keydown(root, { key: 'z', metaKey: true });
+		await tick();
+		expect(screen.component.getValue()).toBe(SOURCE);
+
+		keydown(root, { key: 'y', ctrlKey: true });
+		await tick();
+		expect(screen.component.getValue()).toBe(`${SOURCE} more`);
+	});
+
+	it('coalesces a typing burst into one undo step', async () => {
+		const screen = render(ChatFormContenteditableHarness, { value: 'abc' });
+		await tick();
+
+		const root = editableIn(screen.container);
+		type(root, 'd');
+		type(root, 'e');
+		await tick();
+		expect(screen.component.getValue()).toBe('abcde');
+
+		keydown(root, { key: 'z', ctrlKey: true });
+		await tick();
+		expect(screen.component.getValue()).toBe('abc');
+	});
+
+	it('keeps a newline as its own undo step', async () => {
+		const screen = render(ChatFormContenteditableHarness, { value: 'abc' });
+		await tick();
+
+		const root = editableIn(screen.container);
+		type(root, 'd');
+		type(root, '\n', 'insertLineBreak');
+		await tick();
+		expect(screen.component.getValue()).toBe('abcd\n');
+
+		keydown(root, { key: 'z', ctrlKey: true });
+		await tick();
+		expect(screen.component.getValue()).toBe('abcd');
+
+		keydown(root, { key: 'z', ctrlKey: true });
+		await tick();
+		expect(screen.component.getValue()).toBe('abc');
+	});
+
+	it('is a no-op when there is nothing to undo', async () => {
+		const screen = render(ChatFormContenteditableHarness, { value: 'abc' });
+		await tick();
+
+		const root = editableIn(screen.container);
+		const event = keydown(root, { key: 'z', ctrlKey: true });
+		await tick();
+
+		expect(event.defaultPrevented).toBe(true);
+		expect(screen.component.getValue()).toBe('abc');
+	});
+
+	it('abandons the redo branch after a fresh edit', async () => {
+		const screen = render(ChatFormContenteditableHarness, { value: 'abc' });
+		await tick();
+
+		const root = editableIn(screen.container);
+		type(root, 'd');
+		await tick();
+		keydown(root, { key: 'z', ctrlKey: true });
+		await tick();
+		expect(screen.component.getValue()).toBe('abc');
+
+		type(root, 'e');
+		await tick();
+		keydown(root, { key: 'z', ctrlKey: true, shiftKey: true });
+		await tick();
+		expect(screen.component.getValue()).toBe('abce');
+	});
+});
+
+describe('ChatFormContenteditable Tab key', () => {
+	it('does not trap Tab (focus can leave the editable)', async () => {
+		const screen = render(ChatFormContenteditableHarness, { value: SOURCE });
+		await tick();
+
+		const root = editableIn(screen.container);
+		const event = keydown(root, { key: 'Tab' });
+
+		expect(event.defaultPrevented).toBe(false);
+	});
+});

tools/ui/tests/client/chat-form-contenteditable.svelte.test.ts

@@ -0,0 +1,701 @@
+// Guards the clipboard contract of the chat-form contenteditable:
+// copy/cut expose the markdown SOURCE of the selection (each badge
+// contributes its full `[name](file://...)` link) and pasting such
+// markdown re-renders the badges.
+
+import { describe, it, expect, vi } from 'vitest';
+import { render } from 'vitest-browser-svelte';
+import { userEvent } from 'vitest/browser';
+import { tick } from 'svelte';
+import { rangeToTextOffset, serializeContent, textOffsetToRange } from '$lib/utils';
+import ChatFormContenteditable from '$lib/components/app/chat/ChatForm/ChatFormContenteditable.svelte';
+
+const SOURCE = 'hello [docs](file:///a/b) world';
+const BADGE_SELECTOR = '[data-mention-badge="true"]';
+
+function editableIn(container: HTMLElement): HTMLElement {
+	const el = container.querySelector('[role="textbox"]');
+	if (!(el instanceof HTMLElement)) throw new Error('contenteditable not rendered');
+	return el;
+}
+
+function setSelection(root: HTMLElement, place: (range: Range, root: HTMLElement) => void) {
+	const range = document.createRange();
+	place(range, root);
+	const selection = window.getSelection();
+	if (!selection) throw new Error('no selection');
+	selection.removeAllRanges();
+	selection.addRange(range);
+}
+
+function clipboardEvent(type: 'copy' | 'cut' | 'paste', text = '') {
+	const data = new DataTransfer();
+	if (text) data.setData('text/plain', text);
+	const event = new ClipboardEvent(type, { clipboardData: data, bubbles: true, cancelable: true });
+	return { event, data };
+}
+
+describe('ChatFormContenteditable clipboard', () => {
+	it('copy exposes the markdown source of the selection', async () => {
+		const { container } = render(ChatFormContenteditable, { value: SOURCE });
+		await tick();
+
+		const root = editableIn(container);
+		setSelection(root, (range) => range.selectNodeContents(root));
+
+		const { event, data } = clipboardEvent('copy');
+		root.dispatchEvent(event);
+
+		expect(event.defaultPrevented).toBe(true);
+		expect(data.getData('text/plain')).toBe(SOURCE);
+	});
+
+	it('cut exposes the markdown source and removes the slice', async () => {
+		const { container } = render(ChatFormContenteditable, { value: SOURCE });
+		await tick();
+
+		const root = editableIn(container);
+		setSelection(root, (range) => {
+			const badge = root.querySelector(BADGE_SELECTOR);
+			if (!badge) throw new Error('badge not rendered');
+			range.setStartBefore(badge);
+			range.setEndAfter(badge);
+		});
+
+		const { event, data } = clipboardEvent('cut');
+		root.dispatchEvent(event);
+
+		expect(event.defaultPrevented).toBe(true);
+		expect(data.getData('text/plain')).toBe('[docs](file:///a/b)');
+		expect(root.querySelector(BADGE_SELECTOR)).toBeNull();
+		expect(root.textContent).toBe('hello  world');
+	});
+
+	it('paste of markdown mention links re-renders badges', async () => {
+		const { container } = render(ChatFormContenteditable, { value: 'hello ' });
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+		setSelection(root, (range) => {
+			range.selectNodeContents(root);
+			range.collapse(false);
+		});
+
+		const { event } = clipboardEvent('paste', '[docs](file:///a/b) world');
+		root.dispatchEvent(event);
+		await tick();
+
+		expect(event.defaultPrevented).toBe(true);
+		const badge = root.querySelector(BADGE_SELECTOR);
+		expect(badge).not.toBeNull();
+		expect(badge!.getAttribute('data-mention-name')).toBe('docs');
+		expect(root.textContent).toContain('world');
+	});
+
+	it('paste without mention links keeps the DOM untouched', async () => {
+		const { container } = render(ChatFormContenteditable, { value: 'hello ' });
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+		setSelection(root, (range) => {
+			range.selectNodeContents(root);
+			range.collapse(false);
+		});
+		const firstChild = root.firstChild;
+
+		const { event } = clipboardEvent('paste', 'plain text');
+		root.dispatchEvent(event);
+		await tick();
+
+		expect(event.defaultPrevented).toBe(true);
+		expect(root.querySelector(BADGE_SELECTOR)).toBeNull();
+		// no rebuild: the live text node is the same instance
+		expect(root.firstChild).toBe(firstChild);
+	});
+});
+
+describe('ChatFormContenteditable code spans', () => {
+	it('renders inline code from the initial value', async () => {
+		const { container } = render(ChatFormContenteditable, { value: 'run `npm test` now' });
+		await tick();
+
+		const root = editableIn(container);
+		const code = root.querySelector('code[data-code-token="inline"]');
+		expect(code).not.toBeNull();
+		expect(code!.textContent).toBe('`npm test`');
+	});
+
+	it('renders a fenced code block with a language', async () => {
+		const source = 'before\n```js\nconst a = 1;\n```\nafter';
+		const { container } = render(ChatFormContenteditable, { value: source });
+		await tick();
+
+		const root = editableIn(container);
+		const code = root.querySelector('code[data-code-token="block"]');
+		expect(code).not.toBeNull();
+		expect(code!.textContent).toBe('```js\nconst a = 1;\n```');
+	});
+
+	it('copy exposes the markdown source of a selection spanning code', async () => {
+		const source = 'run `npm test` now';
+		const { container } = render(ChatFormContenteditable, { value: source });
+		await tick();
+
+		const root = editableIn(container);
+		setSelection(root, (range) => range.selectNodeContents(root));
+
+		const { event, data } = clipboardEvent('copy');
+		root.dispatchEvent(event);
+
+		expect(event.defaultPrevented).toBe(true);
+		expect(data.getData('text/plain')).toBe(source);
+	});
+
+	it('paste of a code span renders the styled element', async () => {
+		const { container } = render(ChatFormContenteditable, { value: 'run ' });
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+		setSelection(root, (range) => {
+			range.selectNodeContents(root);
+			range.collapse(false);
+		});
+
+		const { event } = clipboardEvent('paste', '`npm test` now');
+		root.dispatchEvent(event);
+		await tick();
+
+		expect(event.defaultPrevented).toBe(true);
+		const code = root.querySelector('code[data-code-token="inline"]');
+		expect(code).not.toBeNull();
+		expect(code!.textContent).toBe('`npm test`');
+		expect(root.textContent).toContain('now');
+	});
+
+	it('highlights a fenced block content and stays byte-exact', async () => {
+		const source = '```js\nconst a = 1;\n```';
+		const { container } = render(ChatFormContenteditable, { value: source });
+		await tick();
+
+		const root = editableIn(container);
+		const code = root.querySelector('code[data-code-token="block"]');
+		expect(code).not.toBeNull();
+		expect(code!.querySelector('.hljs-keyword')).not.toBeNull();
+		expect(code!.textContent).toBe(source);
+	});
+
+	it('does not highlight inline code', async () => {
+		const { container } = render(ChatFormContenteditable, { value: 'run `const` now' });
+		await tick();
+
+		const root = editableIn(container);
+		expect(root.querySelector('[class*="hljs-"]')).toBeNull();
+	});
+});
+
+describe('ChatFormContenteditable code block escape hatches', () => {
+	const BLOCK_SOURCE = '```js\nconst a = 1;\n```';
+	const BLOCK_SELECTOR = 'code[data-code-token="block"]';
+
+	function blockIn(root: HTMLElement): HTMLElement {
+		const el = root.querySelector(BLOCK_SELECTOR);
+		if (!(el instanceof HTMLElement)) throw new Error('code block not rendered');
+		return el;
+	}
+
+	// Caret at the very start/end of the block's text (across highlight spans)
+	function placeCaretInBlock(root: HTMLElement, where: 'start' | 'end') {
+		const code = blockIn(root);
+		const walker = document.createTreeWalker(code, NodeFilter.SHOW_TEXT);
+		let target: Node | null = null;
+		for (let n = walker.nextNode(); n; n = walker.nextNode()) {
+			target = where === 'start' ? (target ?? n) : n;
+		}
+		if (!target) throw new Error('no text inside code block');
+		setSelection(root, (range) => {
+			range.setStart(target!, where === 'start' ? 0 : (target!.textContent ?? '').length);
+			range.collapse(true);
+		});
+	}
+
+	function caretContainer(): Node {
+		const selection = window.getSelection();
+		if (!selection || selection.rangeCount === 0) throw new Error('no selection');
+		return selection.getRangeAt(0).startContainer;
+	}
+
+	it('pads a trailing code block with a br hatch that stays invisible to copy', async () => {
+		const { container } = render(ChatFormContenteditable, { value: BLOCK_SOURCE });
+		await tick();
+
+		const root = editableIn(container);
+		// no permanent empty line above a leading block
+		expect(root.firstChild).toBe(blockIn(root));
+		expect(root.lastChild?.nodeName).toBe('BR');
+
+		setSelection(root, (range) => range.selectNodeContents(root));
+		const { event, data } = clipboardEvent('copy');
+		root.dispatchEvent(event);
+
+		expect(data.getData('text/plain')).toBe(BLOCK_SOURCE);
+	});
+
+	it('escapes a trailing code block with ArrowDown and types after it', async () => {
+		const { container } = render(ChatFormContenteditable, { value: BLOCK_SOURCE });
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+		placeCaretInBlock(root, 'end');
+
+		await userEvent.keyboard('{ArrowDown}');
+		expect(blockIn(root).contains(caretContainer())).toBe(false);
+
+		await userEvent.keyboard('x');
+		await tick();
+
+		expect(blockIn(root).textContent).toBe(BLOCK_SOURCE);
+		// the DOM holds no separator newline (it would render as a
+		// phantom empty line); serialization synthesizes it so the
+		// markdown source keeps the text below the block
+		expect(root.textContent).toBe(BLOCK_SOURCE + 'x');
+		expect(serializeContent(root)).toBe(BLOCK_SOURCE + '\nx');
+		// the stale trailing hatch is removed once real text follows the block
+		expect(root.lastChild?.nodeName).not.toBe('BR');
+	});
+
+	it('escapes a leading code block with ArrowUp and types before it', async () => {
+		const { container } = render(ChatFormContenteditable, { value: BLOCK_SOURCE });
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+		placeCaretInBlock(root, 'start');
+
+		await userEvent.keyboard('{ArrowUp}');
+		expect(blockIn(root).contains(caretContainer())).toBe(false);
+		// the transient hatch line exists while the caret sits on it
+		expect(root.firstChild?.nodeName).toBe('BR');
+
+		await userEvent.keyboard('y');
+		await tick();
+
+		expect(blockIn(root).textContent).toBe(BLOCK_SOURCE);
+		expect(root.textContent).toBe('y' + BLOCK_SOURCE);
+		expect(serializeContent(root)).toBe('y\n' + BLOCK_SOURCE);
+		// the typed text consumed the hatch
+		expect(root.firstChild?.nodeName).not.toBe('BR');
+	});
+
+	it('escapes a leading code block with ArrowLeft from its first character', async () => {
+		const { container } = render(ChatFormContenteditable, { value: BLOCK_SOURCE });
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+		placeCaretInBlock(root, 'start');
+
+		await userEvent.keyboard('{ArrowLeft}');
+		expect(blockIn(root).contains(caretContainer())).toBe(false);
+		expect(root.firstChild?.nodeName).toBe('BR');
+	});
+
+	it('removes the transient leading hatch when the caret moves back into the block', async () => {
+		const { container } = render(ChatFormContenteditable, { value: BLOCK_SOURCE });
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+		placeCaretInBlock(root, 'start');
+
+		await userEvent.keyboard('{ArrowUp}');
+		expect(root.firstChild?.nodeName).toBe('BR');
+
+		await userEvent.keyboard('{ArrowDown}');
+		await tick();
+
+		expect(blockIn(root).contains(caretContainer())).toBe(true);
+		expect(root.firstChild).toBe(blockIn(root));
+	});
+
+	it('extends the selection out of the block with Shift+ArrowDown', async () => {
+		const { container } = render(ChatFormContenteditable, { value: BLOCK_SOURCE });
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+		placeCaretInBlock(root, 'end');
+
+		await userEvent.keyboard('{Shift>}{ArrowDown}{/Shift}');
+
+		const selection = window.getSelection();
+		expect(selection).not.toBeNull();
+		expect(selection!.isCollapsed).toBe(false);
+		expect(blockIn(root).contains(selection!.getRangeAt(0).endContainer)).toBe(false);
+	});
+
+	it('line-separates text typed right after the closing fence', async () => {
+		const { container } = render(ChatFormContenteditable, { value: BLOCK_SOURCE });
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+		placeCaretInBlock(root, 'end');
+
+		// no arrow keys: the caret sits at the block's end edge, where the
+		// post-rebuild restore lands it, and the typed text renders on the
+		// line below the block
+		await userEvent.keyboard('x');
+		await tick();
+
+		// the text stays on the caret's line in the DOM (no phantom empty
+		// line); the source gets the separator newline
+		expect(root.textContent).toBe(BLOCK_SOURCE + 'x');
+		expect(serializeContent(root)).toBe(BLOCK_SOURCE + '\nx');
+	});
+
+	it('does not double the newline when Shift+Enter already added one', async () => {
+		const { container } = render(ChatFormContenteditable, { value: BLOCK_SOURCE });
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+		placeCaretInBlock(root, 'end');
+
+		await userEvent.keyboard('{Shift>}{Enter}{/Shift}');
+		await userEvent.keyboard('x');
+		await tick();
+
+		expect(root.textContent).toBe(BLOCK_SOURCE + 'x');
+		expect(serializeContent(root)).toBe(BLOCK_SOURCE + '\nx');
+	});
+
+	it('moves a caret stuck before the inserted newline onto the new line', async () => {
+		const { container } = render(ChatFormContenteditable, {
+			value: BLOCK_SOURCE + '\ntext after the code block'
+		});
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+
+		// post-break DOM some browsers produce: the inserted newline plus
+		// the artificial trailing one, with the caret stuck BEFORE the
+		// inserted one (visually at the end of the old line)
+		root.appendChild(document.createTextNode('\n'));
+		root.appendChild(document.createTextNode('\n'));
+		setSelection(root, (range) => {
+			range.setStart(root.childNodes[2], 0);
+			range.collapse(true);
+		});
+
+		root.dispatchEvent(new InputEvent('input', { inputType: 'insertLineBreak', bubbles: true }));
+		await tick();
+
+		const selection = window.getSelection();
+		expect(rangeToTextOffset(root, selection!.getRangeAt(0))).toBe(
+			(BLOCK_SOURCE + '\ntext after the code block\n').length
+		);
+		expect(serializeContent(root)).toBe(BLOCK_SOURCE + '\ntext after the code block\n\n');
+	});
+
+	it('appends the artificial trailing newline when the browser did not add one', async () => {
+		const { container } = render(ChatFormContenteditable, {
+			value: BLOCK_SOURCE + '\ntext after the code block'
+		});
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+
+		// post-break DOM some browsers produce: a lone trailing \n (or a
+		// <br> the hatch sync strips). Collapsed by the renderer, so the
+		// caret looks stuck on the old line and the next typed character
+		// would consume the newline.
+		root.appendChild(document.createTextNode('\n'));
+		setSelection(root, (range) => {
+			range.setStart(root.childNodes[2], 1);
+			range.collapse(true);
+		});
+
+		root.dispatchEvent(new InputEvent('input', { inputType: 'insertLineBreak', bubbles: true }));
+		await tick();
+
+		const selection = window.getSelection();
+		expect(rangeToTextOffset(root, selection!.getRangeAt(0))).toBe(
+			(BLOCK_SOURCE + '\ntext after the code block\n').length
+		);
+		expect(serializeContent(root)).toBe(BLOCK_SOURCE + '\ntext after the code block\n\n');
+	});
+
+	it('lands the caret on the new line with a single Shift+Enter after text below a block', async () => {
+		const { container } = render(ChatFormContenteditable, {
+			value: BLOCK_SOURCE + '\ntext after the code block'
+		});
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+		setSelection(root, (range) => {
+			const text = root.childNodes[1];
+			range.setStart(text, (text.textContent ?? '').length);
+			range.collapse(true);
+		});
+
+		await userEvent.keyboard('{Shift>}{Enter}{/Shift}');
+		await tick();
+
+		const selection = window.getSelection();
+		expect(rangeToTextOffset(root, selection!.getRangeAt(0))).toBe(
+			(BLOCK_SOURCE + '\ntext after the code block\n').length
+		);
+		expect(serializeContent(root)).toBe(BLOCK_SOURCE + '\ntext after the code block\n\n');
+
+		// the next typed character lands on the new line
+		await userEvent.keyboard('x');
+		await tick();
+		expect(serializeContent(root)).toBe(BLOCK_SOURCE + '\ntext after the code block\nx');
+	});
+
+	it('lets Backspace at the text start move into the block without a source fight', async () => {
+		const { container } = render(ChatFormContenteditable, { value: BLOCK_SOURCE });
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+		placeCaretInBlock(root, 'end');
+
+		await userEvent.keyboard('{ArrowDown}');
+		await userEvent.keyboard('create');
+		await tick();
+		expect(serializeContent(root)).toBe(BLOCK_SOURCE + '\ncreate');
+
+		// Backspace at the start of the text line: the separator newline
+		// is structural (synthesized while text follows the block), so
+		// the caret just moves to the block's edge - nothing is re-added
+		await userEvent.keyboard('{Home}');
+		await userEvent.keyboard('{Backspace}');
+		await tick();
+
+		expect(serializeContent(root)).toBe(BLOCK_SOURCE + '\ncreate');
+		expect(caretContainer()).toBe(root);
+	});
+
+	it('lets forward Delete eat the text after a block normally', async () => {
+		const { container } = render(ChatFormContenteditable, { value: BLOCK_SOURCE });
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+		placeCaretInBlock(root, 'end');
+
+		await userEvent.keyboard('{ArrowDown}');
+		await userEvent.keyboard('create');
+		await tick();
+
+		await userEvent.keyboard('{Home}');
+		await userEvent.keyboard('{Delete}');
+		await tick();
+
+		expect(serializeContent(root)).toBe(BLOCK_SOURCE + '\nreate');
+	});
+
+	it('renders text after a block without a phantom empty line', async () => {
+		const { container } = render(ChatFormContenteditable, {
+			value: BLOCK_SOURCE + '\nhello'
+		});
+		await tick();
+
+		const root = editableIn(container);
+		expect(root.textContent).toBe(BLOCK_SOURCE + 'hello');
+		expect(serializeContent(root)).toBe(BLOCK_SOURCE + '\nhello');
+	});
+
+	it('keeps an intentional blank line after a block out of the separator', async () => {
+		const { container } = render(ChatFormContenteditable, {
+			value: BLOCK_SOURCE + '\n\nhello'
+		});
+		await tick();
+
+		const root = editableIn(container);
+		expect(root.textContent).toBe(BLOCK_SOURCE + '\nhello');
+		expect(serializeContent(root)).toBe(BLOCK_SOURCE + '\n\nhello');
+	});
+
+	it('re-highlights while typing inside a block and keeps the caret', async () => {
+		const { container } = render(ChatFormContenteditable, { value: BLOCK_SOURCE });
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+
+		// caret at the start of the block content (after the opening fence)
+		setSelection(root, (range) => {
+			const target = textOffsetToRange(root, 6);
+			range.setStart(target.startContainer, target.startOffset);
+			range.collapse(true);
+		});
+
+		await userEvent.keyboard('x');
+		await tick();
+
+		const code = blockIn(root);
+		expect(serializeContent(root)).toBe('```js\nxconst a = 1;\n```');
+		expect(code.textContent).toBe('```js\nxconst a = 1;\n```');
+		expect(code.querySelector('.hljs-number')).not.toBeNull();
+
+		const selection = window.getSelection();
+		expect(code.contains(selection!.getRangeAt(0).startContainer)).toBe(true);
+		expect(rangeToTextOffset(root, selection!.getRangeAt(0))).toBe(7);
+	});
+});
+
+describe('ChatFormContenteditable Enter in code blocks', () => {
+	const BLOCK_SOURCE = '```js\nconst a = 1;\n```';
+
+	it('adds a line instead of submitting on plain Enter inside a block', async () => {
+		const onKeydown = vi.fn();
+		const { container } = render(ChatFormContenteditable, {
+			value: BLOCK_SOURCE,
+			onKeydown
+		});
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+
+		// caret at the start of the block content (after the opening fence)
+		setSelection(root, (range) => {
+			const target = textOffsetToRange(root, 6);
+			range.setStart(target.startContainer, target.startOffset);
+			range.collapse(true);
+		});
+
+		await userEvent.keyboard('{Enter}');
+		await tick();
+
+		// consumed locally: the parent's submit handler never sees it
+		expect(onKeydown).not.toHaveBeenCalled();
+		expect(serializeContent(root)).toBe('```js\n\nconst a = 1;\n```');
+
+		const code = root.querySelector('code[data-code-token="block"]');
+		const selection = window.getSelection();
+		expect(code!.contains(selection!.getRangeAt(0).startContainer)).toBe(true);
+		expect(rangeToTextOffset(root, selection!.getRangeAt(0))).toBe(7);
+	});
+
+	it('adds a line after a still-open fence (no closing ``` yet)', async () => {
+		const onKeydown = vi.fn();
+		const { container } = render(ChatFormContenteditable, {
+			value: '```js\nconst a = 1;',
+			onKeydown
+		});
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+
+		// caret at the start of the block content (after the opening fence)
+		setSelection(root, (range) => {
+			const target = textOffsetToRange(root, 6);
+			range.setStart(target.startContainer, target.startOffset);
+			range.collapse(true);
+		});
+
+		await userEvent.keyboard('{Enter}');
+		await tick();
+
+		expect(onKeydown).not.toHaveBeenCalled();
+		expect(serializeContent(root)).toBe('```js\n\nconst a = 1;');
+	});
+
+	it('forwards plain Enter to the parent when the caret is outside a block', async () => {
+		const onKeydown = vi.fn();
+		const { container } = render(ChatFormContenteditable, {
+			value: BLOCK_SOURCE + '\nafter',
+			onKeydown
+		});
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+		setSelection(root, (range) => {
+			range.selectNodeContents(root);
+			range.collapse(false);
+		});
+
+		await userEvent.keyboard('{Enter}');
+
+		expect(onKeydown).toHaveBeenCalledTimes(1);
+		expect(onKeydown.mock.calls[0][0].defaultPrevented).toBe(false);
+	});
+
+	it('forwards plain Enter on the trailing hatch line after a block', async () => {
+		const onKeydown = vi.fn();
+		const { container } = render(ChatFormContenteditable, {
+			value: BLOCK_SOURCE,
+			onKeydown
+		});
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+		// root-level caret between the block and its trailing br hatch
+		setSelection(root, (range) => {
+			range.setStart(root, 1);
+			range.collapse(true);
+		});
+
+		await userEvent.keyboard('{Enter}');
+
+		expect(onKeydown).toHaveBeenCalledTimes(1);
+	});
+
+	it('forwards Ctrl+Enter inside a block so explicit submit survives', async () => {
+		const onKeydown = vi.fn();
+		const { container } = render(ChatFormContenteditable, {
+			value: BLOCK_SOURCE,
+			onKeydown
+		});
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+		setSelection(root, (range) => {
+			const target = textOffsetToRange(root, 6);
+			range.setStart(target.startContainer, target.startOffset);
+			range.collapse(true);
+		});
+
+		await userEvent.keyboard('{Control>}{Enter}{/Control}');
+
+		expect(onKeydown).toHaveBeenCalledWith(
+			expect.objectContaining({ key: 'Enter', ctrlKey: true })
+		);
+		expect(serializeContent(root)).toBe(BLOCK_SOURCE);
+	});
+
+	it('forwards Enter inside an inline code span', async () => {
+		const onKeydown = vi.fn();
+		const { container } = render(ChatFormContenteditable, {
+			value: 'run `npm test` now',
+			onKeydown
+		});
+		await tick();
+
+		const root = editableIn(container);
+		root.focus();
+		const code = root.querySelector('code[data-code-token="inline"]')!;
+		setSelection(root, (range) => {
+			range.setStart(code.firstChild!, 3);
+			range.collapse(true);
+		});
+
+		await userEvent.keyboard('{Enter}');
+
+		expect(onKeydown).toHaveBeenCalledTimes(1);
+	});
+});

tools/ui/tests/client/chat-form-enter-code-block.svelte.test.ts

@@ -0,0 +1,112 @@
+// Guards the Enter-key contract of the chat form against the
+// fenced-code-block flow: while the caret sits inside a fenced
+// block region - closed, or still OPEN while the user is typing
+// one - plain Enter adds a line instead of submitting the message.
+// The textarea path is covered here end-to-end (the contenteditable
+// consumes the same case locally; see chat-form-contenteditable).
+
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { render } from 'vitest-browser-svelte';
+import { userEvent } from 'vitest/browser';
+import { tick } from 'svelte';
+import { SETTINGS_KEYS } from '$lib/constants/settings-keys';
+import { settingsStore } from '$lib/stores/settings.svelte';
+import ChatFormTestWrapper from './components/ChatFormTestWrapper.svelte';
+
+function textareaIn(container: HTMLElement): HTMLTextAreaElement {
+	const el = container.querySelector('textarea');
+	if (!(el instanceof HTMLTextAreaElement)) throw new Error('textarea not rendered');
+	return el;
+}
+
+describe('ChatForm Enter in code blocks', () => {
+	beforeEach(() => {
+		settingsStore.updateConfig(SETTINGS_KEYS.SEND_ON_ENTER, true);
+	});
+
+	it('adds a line after a still-open fence instead of submitting', async () => {
+		const onSubmit = vi.fn();
+		const { container } = render(ChatFormTestWrapper, { onSubmit });
+		await tick();
+
+		const textarea = textareaIn(container);
+		await userEvent.click(textarea);
+		await userEvent.keyboard('```');
+		await tick();
+
+		await userEvent.keyboard('{Enter}');
+		await tick();
+
+		expect(onSubmit).not.toHaveBeenCalled();
+		expect(textarea.value).toBe('```\n');
+	});
+
+	it('keeps adding lines while the block stays open', async () => {
+		const onSubmit = vi.fn();
+		const { container } = render(ChatFormTestWrapper, { onSubmit });
+		await tick();
+
+		const textarea = textareaIn(container);
+		await userEvent.click(textarea);
+		await userEvent.keyboard('```js');
+		await tick();
+
+		await userEvent.keyboard('{Enter}');
+		await userEvent.keyboard('const a = 1;');
+		await userEvent.keyboard('{Enter}');
+		await tick();
+
+		expect(onSubmit).not.toHaveBeenCalled();
+		expect(textarea.value).toBe('```js\nconst a = 1;\n');
+	});
+
+	it('submits when the caret is before the opening fence', async () => {
+		const onSubmit = vi.fn();
+		const { container } = render(ChatFormTestWrapper, { onSubmit });
+		await tick();
+
+		const textarea = textareaIn(container);
+		await userEvent.click(textarea);
+		await userEvent.keyboard('```');
+		await tick();
+
+		textarea.setSelectionRange(0, 0);
+
+		await userEvent.keyboard('{Enter}');
+		await tick();
+
+		expect(onSubmit).toHaveBeenCalledTimes(1);
+	});
+
+	it('submits on Enter outside a code block', async () => {
+		const onSubmit = vi.fn();
+		const { container } = render(ChatFormTestWrapper, { onSubmit });
+		await tick();
+
+		const textarea = textareaIn(container);
+		await userEvent.click(textarea);
+		await userEvent.keyboard('hello');
+		await tick();
+
+		await userEvent.keyboard('{Enter}');
+		await tick();
+
+		expect(onSubmit).toHaveBeenCalledTimes(1);
+	});
+
+	it('submits on Ctrl+Enter even inside a code block', async () => {
+		const onSubmit = vi.fn();
+		const { container } = render(ChatFormTestWrapper, { onSubmit });
+		await tick();
+
+		const textarea = textareaIn(container);
+		await userEvent.click(textarea);
+		await userEvent.keyboard('```');
+		await tick();
+
+		await userEvent.keyboard('{Control>}{Enter}{/Control}');
+		await tick();
+
+		expect(onSubmit).toHaveBeenCalledTimes(1);
+	});
+});

tools/ui/tests/client/components/ChatFormContenteditableHarness.svelte

@@ -0,0 +1,27 @@
+<script lang="ts">
+	import { untrack } from 'svelte';
+	import ChatFormContenteditable from '$lib/components/app/chat/ChatForm/ChatFormContenteditable.svelte';
+
+	interface Props {
+		value?: string;
+	}
+
+	let { value: initial = '' }: Props = $props();
+
+	let value = $state(untrack(() => initial));
+	let inputRef: ChatFormContenteditable | undefined = $state(undefined);
+
+	export function getValue() {
+		return value;
+	}
+
+	export function getCaretOffset() {
+		return inputRef?.getCaretOffset();
+	}
+
+	export function setCaretOffset(offset: number) {
+		inputRef?.setCaretOffset(offset);
+	}
+</script>
+
+<ChatFormContenteditable bind:this={inputRef} bind:value />

tools/ui/tests/client/components/ChatFormTestWrapper.svelte

@@ -0,0 +1,12 @@
+<script lang="ts">
+	import * as Tooltip from '$lib/components/ui/tooltip';
+	import ChatForm from '$lib/components/app/chat/ChatForm/ChatForm.svelte';
+
+	let { onSubmit }: { onSubmit?: () => void } = $props();
+
+	let value = $state('');
+</script>
+
+<Tooltip.Provider>
+	<ChatForm bind:value {onSubmit} />
+</Tooltip.Provider>

tools/ui/tests/unit/code.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'vitest';
-import { highlightCode, trimCodePadding } from '$lib/utils/code';
+import { highlightCode, splitGluedClosingCodeFences, trimCodePadding } from '$lib/utils/code';
 
 describe('trimCodePadding', () => {
 	it('removes a single leading newline', () => {
@@ -101,3 +101,38 @@ describe('highlightCode', () => {
 		expect(html).toBe('<script>a && b</script>');
 	});
 });
+
+describe('splitGluedClosingCodeFences', () => {
+	it('splits text glued to a closing fence onto its own line', () => {
+		const input = "```ts\nlet foo = 'bar';\n```create this file on [Desktop](file:///a/b/)";
+		expect(splitGluedClosingCodeFences(input)).toBe(
+			"```ts\nlet foo = 'bar';\n```\ncreate this file on [Desktop](file:///a/b/)"
+		);
+	});
+
+	it('leaves a well-formed code block untouched', () => {
+		const input = "```ts\nlet foo = 'bar';\n```\ncreate this file on [Desktop](file:///a/b/)";
+		expect(splitGluedClosingCodeFences(input)).toBe(input);
+	});
+
+	it('leaves content without fences untouched', () => {
+		expect(splitGluedClosingCodeFences('hello world')).toBe('hello world');
+	});
+
+	it('keeps nested markdown fences inside a block intact', () => {
+		const input = '```md\n# Example\n```python\nprint(1)\n```\n```';
+		expect(splitGluedClosingCodeFences(input)).toBe(input);
+	});
+
+	it('splits every glued closing fence when several blocks are present', () => {
+		const input = '```ts\na\n```first words\n\n```js\nb\n```second words';
+		expect(splitGluedClosingCodeFences(input)).toBe(
+			'```ts\na\n```\nfirst words\n\n```js\nb\n```\nsecond words'
+		);
+	});
+
+	it('leaves a still-open fence untouched', () => {
+		const input = '```ts\nlet foo = 1;';
+		expect(splitGluedClosingCodeFences(input)).toBe(input);
+	});
+});

tools/ui/tests/unit/contenteditable-tokenizer.test.ts

@@ -0,0 +1,205 @@
+import { describe, expect, it } from 'vitest';
+import { containsCodeSpan, isOffsetInCodeBlock, tokenizeContent } from '$lib/utils';
+
+describe('tokenizeContent', () => {
+	it('tokenizes a plain text buffer with no badges', () => {
+		expect(tokenizeContent('hello world')).toEqual([{ kind: 'text', text: 'hello world' }]);
+	});
+
+	it('tokenizes a single badge', () => {
+		expect(tokenizeContent('[docs](file:///a/b)')).toEqual([
+			{ kind: 'badge', name: 'docs', path: '/a/b' }
+		]);
+	});
+
+	it('tokenizes text around a single badge', () => {
+		expect(tokenizeContent('hello [docs](file:///a/b) world')).toEqual([
+			{ kind: 'text', text: 'hello ' },
+			{ kind: 'badge', name: 'docs', path: '/a/b' },
+			{ kind: 'text', text: ' world' }
+		]);
+	});
+
+	it('tokenizes adjacent badges as separate tokens', () => {
+		expect(tokenizeContent('[a](file:///x)[b](file:///y)')).toEqual([
+			{ kind: 'badge', name: 'a', path: '/x' },
+			{ kind: 'badge', name: 'b', path: '/y' }
+		]);
+	});
+
+	it('leaves non-file links untouched in the stream', () => {
+		expect(tokenizeContent('see [foo](https://example.com) for details')).toEqual([
+			{ kind: 'text', text: 'see [foo](https://example.com) for details' }
+		]);
+	});
+
+	it('recognizes badges whose path contains spaces (macOS screenshots)', () => {
+		const path = '/Users/allozaur/Desktop/Screenshot 2026-07-28 at 17.21.50.png';
+		const source = `[Screenshot 2026-07-28 at 17.21.50.png](file://${path}) `;
+		expect(tokenizeContent(source)).toEqual([
+			{ kind: 'badge', name: 'Screenshot 2026-07-28 at 17.21.50.png', path },
+			{ kind: 'text', text: ' ' }
+		]);
+	});
+
+	it('recognizes badges whose path lives in the macOS temp folder', () => {
+		const path =
+			'/var/folders/78/j28m7pn57wb34bfjwlskh62h0000gn/T/TemporaryItems/NSIRD_screencaptureui_GD0A2R/Screenshot 2026-07-28 at 17.23.28.png';
+		const source = `[Screenshot 2026-07-28 at 17.23.28.png](file://${path}) `;
+		expect(tokenizeContent(source)).toEqual([
+			{ kind: 'badge', name: 'Screenshot 2026-07-28 at 17.23.28.png', path },
+			{ kind: 'text', text: ' ' }
+		]);
+	});
+
+	it('keeps text around a badge with spaces in the path', () => {
+		const path = '/Users/allozaur/Desktop/Screenshot 2026-07-28 at 17.21.50.png';
+		const source = `see [Screenshot 2026-07-28 at 17.21.50.png](file://${path}) done`;
+		expect(tokenizeContent(source)).toEqual([
+			{ kind: 'text', text: 'see ' },
+			{ kind: 'badge', name: 'Screenshot 2026-07-28 at 17.21.50.png', path },
+			{ kind: 'text', text: ' done' }
+		]);
+	});
+
+	it('recognizes badges whose path contains a close parenthesis (macOS duplicate files)', () => {
+		const path = '/Users/foo/Screenshot (1).png';
+		const source = `[Screenshot (1).png](file://${path}) `;
+		expect(tokenizeContent(source)).toEqual([
+			{ kind: 'badge', name: 'Screenshot (1).png', path },
+			{ kind: 'text', text: ' ' }
+		]);
+	});
+
+	it('recognizes badges whose folder name is wrapped in parentheses', () => {
+		const path = '/Users/foo/Project (Stuff)/main.rs';
+		const source = `[main.rs](file://${path}) `;
+		expect(tokenizeContent(source)).toEqual([
+			{ kind: 'badge', name: 'main.rs', path },
+			{ kind: 'text', text: ' ' }
+		]);
+	});
+
+	it('recognizes adjacent badges back-to-back with no separator', () => {
+		const source = '[a](file:///p)[b](file:///q)';
+		expect(tokenizeContent(source)).toEqual([
+			{ kind: 'badge', name: 'a', path: '/p' },
+			{ kind: 'badge', name: 'b', path: '/q' }
+		]);
+	});
+
+	it('tokenizes inline code with the backticks included', () => {
+		expect(tokenizeContent('run `npm test` now')).toEqual([
+			{ kind: 'text', text: 'run ' },
+			{ kind: 'inlineCode', text: '`npm test`' },
+			{ kind: 'text', text: ' now' }
+		]);
+	});
+
+	it('tokenizes a fenced code block without a language', () => {
+		const source = 'before\n```\nconst a = 1;\n```\nafter';
+		expect(tokenizeContent(source)).toEqual([
+			{ kind: 'text', text: 'before\n' },
+			{ kind: 'codeBlock', text: '```\nconst a = 1;\n```' },
+			{ kind: 'text', text: '\nafter' }
+		]);
+	});
+
+	it('tokenizes a fenced code block with a language', () => {
+		const source = '```js\nconst a = 1;\n```';
+		expect(tokenizeContent(source)).toEqual([
+			{ kind: 'codeBlock', text: '```js\nconst a = 1;\n```' }
+		]);
+	});
+
+	it('prefers the fenced block over inline spans at triple backticks', () => {
+		expect(tokenizeContent('```a``` ```b```')).toEqual([
+			{ kind: 'codeBlock', text: '```a```' },
+			{ kind: 'text', text: ' ' },
+			{ kind: 'codeBlock', text: '```b```' }
+		]);
+	});
+
+	it('leaves an unclosed fence as plain text', () => {
+		expect(tokenizeContent('```js\nconst a = 1;')).toEqual([
+			{ kind: 'text', text: '```js\nconst a = 1;' }
+		]);
+	});
+
+	it('leaves an unclosed inline backtick as plain text', () => {
+		expect(tokenizeContent('run `npm test')).toEqual([{ kind: 'text', text: 'run `npm test' }]);
+	});
+
+	it('does not recognize badges inside code spans', () => {
+		expect(tokenizeContent('`[a](file:///p)`')).toEqual([
+			{ kind: 'inlineCode', text: '`[a](file:///p)`' }
+		]);
+	});
+
+	it('tokenizes badges and code spans side by side', () => {
+		expect(tokenizeContent('[a](file:///p) `x`')).toEqual([
+			{ kind: 'badge', name: 'a', path: '/p' },
+			{ kind: 'text', text: ' ' },
+			{ kind: 'inlineCode', text: '`x`' }
+		]);
+	});
+});
+
+describe('containsCodeSpan', () => {
+	it('detects inline code', () => {
+		expect(containsCodeSpan('run `npm test` now')).toBe(true);
+	});
+
+	it('detects a fenced block with a language', () => {
+		expect(containsCodeSpan('```js\nconst a = 1;\n```')).toBe(true);
+	});
+
+	it('detects a fenced block without a language', () => {
+		expect(containsCodeSpan('```\ncode\n```')).toBe(true);
+	});
+
+	it('ignores unclosed fences and lone backticks', () => {
+		expect(containsCodeSpan('```js\nconst a = 1;')).toBe(false);
+		expect(containsCodeSpan('run `npm test')).toBe(false);
+		expect(containsCodeSpan('``')).toBe(false);
+	});
+
+	it('ignores plain text and mention links', () => {
+		expect(containsCodeSpan('hello world')).toBe(false);
+		expect(containsCodeSpan('[a](file:///p)')).toBe(false);
+	});
+});
+
+describe('isOffsetInCodeBlock', () => {
+	const BLOCK = '```js\nconst a = 1;\n```';
+
+	it('is false with no fences in the buffer', () => {
+		expect(isOffsetInCodeBlock('hello world', 5)).toBe(false);
+		expect(isOffsetInCodeBlock('run `npm test` now', 10)).toBe(false);
+	});
+
+	it('is true right after the opening fence, before any content', () => {
+		expect(isOffsetInCodeBlock('```', 3)).toBe(true);
+		expect(isOffsetInCodeBlock('```js', 5)).toBe(true);
+	});
+
+	it('is true inside a still-open block while it is being typed', () => {
+		const open = '```js\nconst a = 1;';
+		expect(isOffsetInCodeBlock(open, open.length)).toBe(true);
+	});
+
+	it('is true inside a closed block and false outside it', () => {
+		expect(isOffsetInCodeBlock(BLOCK, 6)).toBe(true);
+		expect(isOffsetInCodeBlock(BLOCK, 0)).toBe(false);
+		expect(isOffsetInCodeBlock(BLOCK, BLOCK.length)).toBe(false);
+		expect(isOffsetInCodeBlock(BLOCK + '\nafter', BLOCK.length + 5)).toBe(false);
+	});
+
+	it('toggles per fence across multiple blocks', () => {
+		const two = BLOCK + '\ntext\n' + BLOCK;
+		const secondBlock = two.lastIndexOf(BLOCK);
+		expect(isOffsetInCodeBlock(two, secondBlock - 2)).toBe(false);
+		expect(isOffsetInCodeBlock(two, secondBlock + 6)).toBe(true);
+		expect(isOffsetInCodeBlock(two, two.length)).toBe(false);
+	});
+});

tools/ui/tests/unit/contenteditable-word-jump.test.ts

@@ -0,0 +1,80 @@
+import { describe, expect, it } from 'vitest';
+import { badgeAwareWordJump, leadingBadgeEdgeOffset } from '$lib/utils';
+
+// Layout of `hello [docs](file:///a/b) world foo`:
+//   "hello" 0-4, " " 5, badge 6-24 (length 19), " " 25, "world" 26-30, " " 31, "foo" 32-34
+const BADGE = '[docs](file:///a/b)';
+const SOURCE = `hello ${BADGE} world foo`;
+const BADGE_START = 6;
+const BADGE_END = 25;
+
+describe('badgeAwareWordJump', () => {
+	it('returns null when the buffer has no badge', () => {
+		expect(badgeAwareWordJump('hello world', 0, 'forward')).toBeNull();
+		expect(badgeAwareWordJump('hello world', 11, 'backward')).toBeNull();
+	});
+
+	it('jumps forward onto a badge landing at its end, not the next word', () => {
+		expect(badgeAwareWordJump(SOURCE, BADGE_START, 'forward')).toBe(BADGE_END);
+	});
+
+	it('jumps forward from the space before a badge landing at its end', () => {
+		expect(badgeAwareWordJump(SOURCE, BADGE_START - 1, 'forward')).toBe(BADGE_END);
+	});
+
+	it('jumps backward over a badge landing at its start', () => {
+		expect(badgeAwareWordJump(SOURCE, BADGE_END, 'backward')).toBe(BADGE_START);
+	});
+
+	it('jumps backward from the next word onto the badge start', () => {
+		// caret at the start of "world"
+		expect(badgeAwareWordJump(SOURCE, BADGE_END + 1, 'backward')).toBe(BADGE_START);
+	});
+
+	it('returns null for jumps that cross no badge', () => {
+		// forward over "hello" only
+		expect(badgeAwareWordJump(SOURCE, 0, 'forward')).toBeNull();
+		// backward over "foo" only
+		expect(badgeAwareWordJump(SOURCE, SOURCE.length, 'backward')).toBeNull();
+		// backward away from the badge (over "hello")
+		expect(badgeAwareWordJump(SOURCE, BADGE_START, 'backward')).toBeNull();
+	});
+
+	it('treats a leading badge as one word in both directions', () => {
+		const source = `${BADGE} rest`;
+		expect(badgeAwareWordJump(source, 0, 'forward')).toBe(BADGE.length);
+		expect(badgeAwareWordJump(source, BADGE.length, 'backward')).toBe(0);
+	});
+
+	it('treats adjacent badges as separate words', () => {
+		// each badge is 14 chars: "[a](file:///x)" / "[b](file:///y)"
+		const source = '[a](file:///x)[b](file:///y)';
+		expect(badgeAwareWordJump(source, 0, 'forward')).toBe(14);
+		expect(badgeAwareWordJump(source, 14, 'forward')).toBe(28);
+		expect(badgeAwareWordJump(source, 28, 'backward')).toBe(14);
+		expect(badgeAwareWordJump(source, 14, 'backward')).toBe(0);
+	});
+
+	it('jumps over a badge following punctuation', () => {
+		// "foo," 0-3, " " 4, badge 5-23 (end 24), " bar" 24-27
+		const source = `foo, ${BADGE} bar`;
+		expect(badgeAwareWordJump(source, 0, 'forward')).toBeNull();
+		expect(badgeAwareWordJump(source, 3, 'forward')).toBe(24);
+	});
+});
+
+describe('leadingBadgeEdgeOffset', () => {
+	it('returns 0 when the caret sits exactly at a leading badge end', () => {
+		expect(leadingBadgeEdgeOffset(`${BADGE} rest`, BADGE.length)).toBe(0);
+	});
+
+	it('returns null when the caret is anywhere else', () => {
+		expect(leadingBadgeEdgeOffset(`${BADGE} rest`, 0)).toBeNull();
+		expect(leadingBadgeEdgeOffset(`${BADGE} rest`, BADGE.length + 2)).toBeNull();
+	});
+
+	it('returns null when the buffer does not start with a badge', () => {
+		expect(leadingBadgeEdgeOffset(SOURCE, BADGE_END)).toBeNull();
+		expect(leadingBadgeEdgeOffset('', 0)).toBeNull();
+	});
+});

tools/ui/tests/unit/mention-badge.test.ts

@@ -3,12 +3,12 @@ import {
 	MENTION_BADGE_FILE_ICON_PATHS,
 	MENTION_BADGE_FOLDER_ICON_PATHS,
 	buildMentionInsertion,
+	containsFileMentionLink,
 	decodeFileLinkPath,
 	encodeFileLinkPath,
 	fileMentionLinkRe,
 	getMentionBadgeIconPaths,
-	getMentionBadgeLabel,
+	getMentionBadgeLabel
-	mentionLinkEndingAt
 } from '$lib/utils';
 import { FileMentionEntryType } from '$lib/enums';
 
@@ -35,6 +35,7 @@ describe('encodeFileLinkPath', () => {
 describe('fileMentionLinkRe', () => {
 	it('matches a standard mention link', () => {
 		expect(fileMentionLinkRe().test('[docs](file:///a/b)')).toBe(true);
+		expect(containsFileMentionLink('[docs](file:///a/b)')).toBe(true);
 	});
 
 	it('does not match non-file links', () => {
@@ -171,32 +172,3 @@ describe('buildMentionInsertion', () => {
 		expect(buildMentionInsertion(file('/a/b.txt', 'b.txt'), 'x', { start: 2, end: 1 })).toBeNull();
 	});
 });
-
-describe('mentionLinkEndingAt', () => {
-	const LINK = '[docs](file:///a/b)';
-
-	it('returns the extent when the caret is exactly at the link end', () => {
-		expect(mentionLinkEndingAt(`see ${LINK} here`, 4 + LINK.length)).toEqual({
-			start: 4,
-			end: 4 + LINK.length
-		});
-	});
-
-	it('returns null when the caret is inside or past the link', () => {
-		expect(mentionLinkEndingAt(LINK, LINK.length - 1)).toBeNull();
-		expect(mentionLinkEndingAt(`${LINK} `, LINK.length + 1)).toBeNull();
-	});
-
-	it('returns null for non-file links and plain text', () => {
-		expect(mentionLinkEndingAt('[foo](https://example.com)', 26)).toBeNull();
-		expect(mentionLinkEndingAt('plain', 5)).toBeNull();
-	});
-
-	it('picks the link that ends at the caret when several exist', () => {
-		const value = `${LINK} and ${LINK}`;
-		expect(mentionLinkEndingAt(value, value.length)).toEqual({
-			start: value.length - LINK.length,
-			end: value.length
-		});
-	});
-});

tools/ui/tests/unit/source-history.test.ts

@@ -0,0 +1,65 @@
+import { describe, expect, it } from 'vitest';
+import { SourceHistory } from '$lib/utils';
+
+describe('SourceHistory', () => {
+	it('coalesces pushes inside the group window into one undo step', () => {
+		const h = new SourceHistory(100, 800);
+		h.push({ value: '', caret: 0 }, 1000);
+		h.push({ value: 'a', caret: 1 }, 1200);
+		h.push({ value: 'ab', caret: 2 }, 1500);
+
+		expect(h.undo({ value: 'abc', caret: 3 })).toEqual({ value: '', caret: 0 });
+		expect(h.undo({ value: '', caret: 0 })).toBeNull();
+	});
+
+	it('starts a new group once the window has passed', () => {
+		const h = new SourceHistory(100, 800);
+		h.push({ value: '', caret: 0 }, 1000);
+		h.push({ value: 'abc', caret: 3 }, 2000);
+
+		expect(h.undo({ value: 'abcdef', caret: 6 })).toEqual({ value: 'abc', caret: 3 });
+		expect(h.undo({ value: 'abc', caret: 3 })).toEqual({ value: '', caret: 0 });
+	});
+
+	it('newGroup forces a separate entry even inside the window', () => {
+		const h = new SourceHistory(100, 800);
+		h.push({ value: '', caret: 0 }, 1000);
+		h.push({ value: 'abc', caret: 3 }, 1100, true);
+
+		expect(h.undo({ value: 'abc\n', caret: 4 })).toEqual({ value: 'abc', caret: 3 });
+		expect(h.undo({ value: 'abc', caret: 3 })).toEqual({ value: '', caret: 0 });
+	});
+
+	it('redo round-trips and a fresh push clears the redo stack', () => {
+		const h = new SourceHistory(100, 800);
+		h.push({ value: '', caret: 0 }, 1000);
+
+		const undone = h.undo({ value: 'abc', caret: 3 });
+		expect(undone).toEqual({ value: '', caret: 0 });
+		expect(h.redo({ value: '', caret: 0 })).toEqual({ value: 'abc', caret: 3 });
+
+		h.undo({ value: 'abc', caret: 3 });
+		h.push({ value: '', caret: 0 }, 5000);
+		expect(h.redo({ value: 'x', caret: 1 })).toBeNull();
+	});
+
+	it('starts a new group on the first edit after an undo', () => {
+		const h = new SourceHistory(100, 800);
+		h.push({ value: '', caret: 0 }, 1000);
+		h.undo({ value: 'abc', caret: 3 });
+
+		h.push({ value: '', caret: 0 }, 1200);
+		expect(h.undo({ value: 'x', caret: 1 })).toEqual({ value: '', caret: 0 });
+	});
+
+	it('evicts the oldest entry past the limit', () => {
+		const h = new SourceHistory(2, 800);
+		h.push({ value: 'one', caret: 0 }, 1000);
+		h.push({ value: 'two', caret: 0 }, 2000);
+		h.push({ value: 'three', caret: 0 }, 3000);
+
+		expect(h.undo({ value: 'cur', caret: 0 })).toEqual({ value: 'three', caret: 0 });
+		expect(h.undo({ value: 'three', caret: 0 })).toEqual({ value: 'two', caret: 0 });
+		expect(h.undo({ value: 'two', caret: 0 })).toBeNull();
+	});
+});