Completes the markdown plugin's deferred v2 items:
1. TOC pane
A third pane to the right of the Toast UI editor lists every heading
in the current document, hierarchically indented by level. Click an
item → editor scrolls to that heading (markdown-mode uses
setSelection + preview scroll; WYSIWYG mode uses DOM text matching;
the target heading flashes briefly via primary-light background).
The TOC re-renders on every editor change (debounced 250ms) so it
stays in sync with edits.
Heading parser supports ATX-style `^#{1,6}\s+` lines, strips inline
markdown emphasis/code/links/strike from the displayed label.
Empty file → "Empty file." Headingless file → "No headings."
2. FS-API writes
Saves now route to whichever source the file came from:
- node.handle + createWritable available → FileSystemWritableFileStream
(local folder picker). The user's chosen file gets overwritten
via the browser's File System Access API.
- node.url + server source → PUT to the server URL (as before).
- zip-virtual file → save disabled (no writable stream from JSZip).
- Anything else → save disabled with a tooltip.
Save status surfaces via the existing toolbar (`Saved 10:42:18`) AND
a shared toast notification ("Saved readme.md" / "Save failed: …")
so the success/failure is visible regardless of whether the user is
looking at the toolbar.
Source-hint chip on the toolbar shows "local" / "server" /
"read-only (inside zip)" so the user knows which write path is
active before they make changes.
CSS additions in browse/css/tree.css for .md-toolbar, .md-split,
.md-editor-host, .md-toc-pane, .toc-list, and the .toc-level-1..6
indentation rules.
A new Playwright test exercises the markdown plugin end-to-end:
mounts the editor on a .md click, asserts the three DOM regions are
visible, verifies the TOC contains the three expected headings from
the test fixture's markdown content, and confirms the source hint
reads "local" for FS-API mode.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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