Add document outline support for Markdown/Quarto files

outline.ts: export NestingLevel so it can be used outside the file.

markdown/document-outline.ts: new enterMarkdownNode function that walks
  the Lezer Markdown syntax tree and extracts ATXHeading1-6 and
  SetextHeading1-2 nodes, mapping them to the same NestingLevel enum
  used by the LaTeX outline (Section→SubSection→SubSubSection…).
  Wrapped in makeProjectionStateField for incremental updates.

markdown/index.ts: register markdownDocumentOutline as a CodeMirror
  extension in the Markdown LanguageSupport so the StateField is active
  whenever a .qmd file is open.

codemirror-outline.tsx: fall back to markdownDocumentOutline when the
  LaTeX documentOutline StateField is not present in the editor state
  (i.e. when the active language is Markdown, not LaTeX).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/web/frontend/js/features/source-editor/components/codemirror-outline.tsx

@@ -1,6 +1,7 @@
 import { useCodeMirrorStateContext } from './codemirror-context'
 import React, { useEffect } from 'react'
 import { documentOutline } from '../languages/latex/document-outline'
+import { markdownDocumentOutline } from '../languages/markdown/document-outline'
 import { ProjectionStatus } from '../utils/tree-operations/projection'
 import useDebounce from '../../../shared/hooks/use-debounce'
 import { useOutlineContext } from '@/features/ide-react/context/outline-context'
@@ -10,7 +11,10 @@ export const CodemirrorOutline = React.memo(function CodemirrorOutline() {
 
   const state = useCodeMirrorStateContext()
   const debouncedState = useDebounce(state, 100)
-  const outlineResult = debouncedState.field(documentOutline, false)
+  // Use whichever outline StateField is active for the current language
+  const outlineResult =
+    debouncedState.field(documentOutline, false) ??
+    debouncedState.field(markdownDocumentOutline, false)
 
   // when the outline projection changes, calculate the flat outline
   useEffect(() => {

services/web/frontend/js/features/source-editor/languages/markdown/document-outline.ts

@@ -0,0 +1,58 @@
+import { EditorState } from '@codemirror/state'
+import { SyntaxNodeRef } from '@lezer/common'
+import {
+  FlatOutlineItem,
+  NestingLevel,
+} from '../../utils/tree-operations/outline'
+import { NodeIntersectsChangeFn } from '../../utils/tree-operations/projection'
+import { makeProjectionStateField } from '../../utils/projection-state-field'
+
+// Map Lezer Markdown node names to outline nesting levels
+const HEADING_LEVELS: Record<string, NestingLevel> = {
+  ATXHeading1: NestingLevel.Section,
+  ATXHeading2: NestingLevel.SubSection,
+  ATXHeading3: NestingLevel.SubSubSection,
+  ATXHeading4: NestingLevel.Paragraph,
+  ATXHeading5: NestingLevel.SubParagraph,
+  ATXHeading6: NestingLevel.SubParagraph,
+  SetextHeading1: NestingLevel.Section,
+  SetextHeading2: NestingLevel.SubSection,
+}
+
+const enterMarkdownNode = (
+  state: EditorState,
+  node: SyntaxNodeRef,
+  items: FlatOutlineItem[],
+  nodeIntersectsChange: NodeIntersectsChangeFn
+): void => {
+  const level = HEADING_LEVELS[node.name]
+  if (level === undefined) return
+
+  if (!nodeIntersectsChange(node)) {
+    // Node unchanged — already present in items from the previous projection
+    return
+  }
+
+  // Collect heading text, skipping the HeaderMark (the leading # characters)
+  let title = ''
+  const cursor = node.node.cursor()
+  if (cursor.firstChild()) {
+    do {
+      if (cursor.name !== 'HeaderMark') {
+        title += state.sliceDoc(cursor.from, cursor.to)
+      }
+    } while (cursor.nextSibling())
+  }
+
+  items.push({
+    line: state.doc.lineAt(node.from).number,
+    toLine: state.doc.lineAt(node.to).number,
+    title: title.trim(),
+    from: node.from,
+    to: node.to,
+    level,
+  } as FlatOutlineItem)
+}
+
+export const markdownDocumentOutline =
+  makeProjectionStateField<FlatOutlineItem>(enterMarkdownNode)

services/web/frontend/js/features/source-editor/languages/markdown/index.ts

@@ -8,6 +8,7 @@ import {
   syntaxHighlighting,
 } from '@codemirror/language'
 import { tags } from '@lezer/highlight'
+import { markdownDocumentOutline } from './document-outline'
 
 export const markdown = () => {
   const { language, support } = markdownLanguage({
@@ -19,6 +20,7 @@ export const markdown = () => {
     support,
     shortcuts(),
     syntaxHighlighting(markdownHighlightStyle),
+    markdownDocumentOutline,
   ])
 }
 

services/web/frontend/js/features/source-editor/utils/tree-operations/outline.ts

@@ -24,7 +24,7 @@ export class FlatOutlineItem extends ProjectionItem {
 export type FlatOutline = FlatOutlineItem[]
 
 /* eslint-disable no-unused-vars */
-enum NestingLevel {
+export enum NestingLevel {
   Book = 1,
   Part = 2,
   Chapter = 3,