feat(browse): CodeMirror YAML editor for the markdown front-matter pane

Replace the raw <textarea> front-matter editor with a CodeMirror 5 instance —
the same editor family the .zddc previewer already uses (and already bundled in
browse, so no added weight). Gains: YAML syntax highlighting, line numbers, and
the shared js-yaml lint gutter; one consistent editing feel across the two YAML
surfaces.

- Mount fmCM (mode:yaml, lineNumbers, lint gutter, readOnly when not writable)
  into a host div; refresh on the next frame so it measures correctly.
- Route every front-matter read/write through fmCM.getValue()/setValue() and
  the change event (sync-on-open, dirty tracking, the rename cue, Cancel, save).
- The old textarea placeholder (recognised front-matter keys) becomes a greyed
  caption under the header whose tooltip carries the full key list — CodeMirror
  5 has no built-in placeholder. Arbitrary keys remain free either way.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

browse/css/tree.css

@@ -896,39 +896,37 @@ body {
 
 /* ── Front matter editor ────────────────────────────────────────────────── */
 .md-fm__body {
-    /* Body cell owns the textarea; sized by the sidebar's grid row. */
+    /* Body cell owns the CodeMirror editor; sized by the sidebar's grid row. */
     padding: 0;
     display: block;
     overflow: hidden;
 }
-.md-fm__textarea {
+/* Recognised-keys caption under the header (tooltip carries the full list). */
-    width: 100%;
+.md-fm__hint {
+    padding: 2px 0.6rem 4px;
+    font-size: 0.72rem;
+    color: var(--text-muted);
+    cursor: help;
+    white-space: nowrap;
+    overflow: hidden;
+    text-overflow: ellipsis;
+}
+/* CodeMirror YAML front-matter editor — fills the body cell + scrolls
+   internally, matching the .zddc previewer's editor styling. */
+.md-fm__editor,
+.md-fm__editor .CodeMirror {
     height: 100%;
-    box-sizing: border-box;
+}
-    margin: 0;
+.md-fm__editor .CodeMirror {
-    padding: 0.4rem 0.6rem;
-    border: 0;
-    background: transparent;
-    color: var(--text);
     font-family: var(--font-mono, ui-monospace, SFMono-Regular, Consolas, monospace);
     font-size: 0.8rem;
     line-height: 1.45;
-    resize: none;
+    background: transparent;
-    outline: none;
+    color: var(--text);
-    white-space: pre;
-    overflow: auto;
-    tab-size: 2;
 }
-.md-fm__textarea::placeholder {
+.md-fm__editor .CodeMirror-gutters {
-    color: var(--text-muted);
+    background: var(--bg-secondary);
-    font-style: italic;
+    border-right: 1px solid var(--border);
-}
-.md-fm__textarea:focus {
-    background: var(--surface-2, rgba(0, 0, 0, 0.025));
-}
-.md-fm__textarea[readonly] {
-    color: var(--text-muted);
-    cursor: not-allowed;
 }
 
 /* Older .md-fm-section / .fm-list / .md-toc-resizer rules were replaced

browse/js/preview-markdown.js

@@ -83,16 +83,22 @@
     var fmPlaceholder = null;
     var fmPlaceholderPromise = null;
 
-    // applyFrontMatterPlaceholder sets the textarea placeholder to the server's
+    // applyFrontMatterHint populates a greyed caption (+ tooltip) with the
-    // recognised-field hint, in server mode only. Async + best-effort: a failed
+    // server's recognised front-matter fields, in server mode only. Async +
-    // fetch leaves the pane blank (no placeholder), never an error.
+    // best-effort: a failed fetch leaves the caption hidden, never an error.
-    function applyFrontMatterPlaceholder(textarea) {
+    // (Replaces the old textarea placeholder — CodeMirror 5 has no built-in
+    // placeholder without an unvendored add-on. Arbitrary keys stay free.)
+    function applyFrontMatterHint(el) {
         var st = window.app && window.app.state;
         if (!st || st.source !== 'server') return;
-        if (fmPlaceholder !== null) {
+        function paint() {
-            textarea.placeholder = fmPlaceholder;
+            if (!el.isConnected) return; // user switched files before resolve
-            return;
+            if (!fmPlaceholder) { el.style.display = 'none'; return; }
+            el.textContent = 'ⓘ Recognised front-matter keys (hover) — any other key is allowed';
+            el.title = fmPlaceholder;
+            el.style.display = '';
         }
+        if (fmPlaceholder !== null) { paint(); return; }
         if (!fmPlaceholderPromise) {
             fmPlaceholderPromise = fetch('/.api/frontmatter', {
                 headers: { 'Accept': 'application/json' },
@@ -101,11 +107,7 @@
               .then(function (j) { fmPlaceholder = (j && j.placeholder) || ''; })
               .catch(function () { fmPlaceholder = ''; });
         }
-        fmPlaceholderPromise.then(function () {
+        fmPlaceholderPromise.then(paint);
-            // Only apply if this textarea is still in the DOM (user may have
-            // switched files before the fetch resolved).
-            if (textarea.isConnected) textarea.placeholder = fmPlaceholder;
-        });
     }
 
     // Lightweight YAML front-matter parser. Same envelope as mdedit's:
@@ -427,22 +429,19 @@
         fmHeader.textContent = 'YAML front matter';
         var fmBody = document.createElement('div');
         fmBody.className = 'md-side__body md-fm__body';
-        var fmTextarea = document.createElement('textarea');
+        // CodeMirror YAML editor host — mounted with the front-matter value
-        fmTextarea.className = 'md-fm__textarea';
+        // once it's computed (sync-on-open) below. Same editor family as the
-        fmTextarea.spellcheck = false;
+        // .zddc previewer: syntax highlighting, line numbers, lint gutter.
-        fmTextarea.autocapitalize = 'off';
+        var fmEditorHost = document.createElement('div');
-        fmTextarea.autocomplete = 'off';
+        fmEditorHost.className = 'md-fm__editor';
-        // Placeholder: in server mode, hint the recognised front-matter keys
+        fmBody.appendChild(fmEditorHost);
-        // (doctype, numbering, …) as greyed text so authors can discover them.
+        // Recognised-keys hint (server mode): a greyed caption under the header
-        // It's placeholder-only — inserts nothing, vanishes on the first
+        // whose tooltip carries the full "key: # hint" template from
-        // keystroke — so arbitrary keys stay free and a file with no front
+        // /.api/frontmatter. Replaces the old textarea placeholder.
-        // matter still renders as a genuinely empty pane. The text is fetched
+        var fmHint = document.createElement('div');
-        // from the server (/.api/frontmatter), the single source of truth, so
+        fmHint.className = 'md-fm__hint';
-        // it never drifts from what the converter honours. file:// mode shows
+        fmHint.style.display = 'none';
-        // no placeholder (conversion is server-only).
+        applyFrontMatterHint(fmHint);
-        fmTextarea.placeholder = '';
-        applyFrontMatterPlaceholder(fmTextarea);
-        fmBody.appendChild(fmTextarea);
         // Rename cue: shown when the author edits an identity field
         // (tracking_number / revision / status / title) away from the
         // filename. The filename owns identity, so the cue offers an explicit
@@ -459,6 +458,7 @@
             + '0.78rem;line-height:1.5;flex-wrap:wrap;align-items:center;gap:6px;'
             + 'display:none;';
         fmSection.appendChild(fmHeader);
+        fmSection.appendChild(fmHint);
         fmSection.appendChild(fmWarn);
         fmSection.appendChild(fmBody);
         sidebar.appendChild(fmSection);
@@ -615,9 +615,32 @@
                 if (Object.prototype.hasOwnProperty.call(fid, ik)) initialParsed.data[ik] = fid[ik];
             }
         }
-        fmTextarea.value = stringifyFrontMatter(initialParsed.data);
+        var syncedFM = stringifyFrontMatter(initialParsed.data);
         var initialHash = await hashContent(assembleContent(onDiskFM, bodyText));
         var writableMode = canSave(node);
+
+        // Front-matter YAML editor — CodeMirror, the same editor family as the
+        // .zddc previewer (syntax highlighting, line numbers, shared js-yaml
+        // lint gutter). Replaces the old <textarea>.
+        var fmCM = window.CodeMirror(fmEditorHost, {
+            value: syncedFM,
+            mode: 'yaml',
+            lineNumbers: true,
+            tabSize: 2,
+            indentUnit: 2,
+            indentWithTabs: false,
+            lineWrapping: true,
+            gutters: ['CodeMirror-lint-markers', 'CodeMirror-linenumbers'],
+            lint: { hasGutters: true },
+            autofocus: false,
+            readOnly: !writableMode
+        });
+        // The yaml lint helper (registered by the .zddc previewer) checks this
+        // to decide the schema layer; a .md node → plain js-yaml parse lint.
+        fmCM._zddcNode = node;
+        // CodeMirror mis-measures when mounted before its pane is laid out;
+        // refresh on the next frame so the gutters + scroll size correctly.
+        requestAnimationFrame(function () { fmCM.refresh(); });
         // autofocus:false keeps the keyboard caret in the tree pane —
         // arrow-key nav can continue through markdown files without
         // diverting into the editor. The user clicks into the editor
@@ -666,7 +689,7 @@
             node: node,
             hash: initialHash,
             tocEl: tocBody,
-            fmEl: fmTextarea,
+            fmEl: fmCM,
             ac: ac,
             // Server version token captured at load — sent as If-Match on
             // save and refreshed from each successful PUT's response ETag.
@@ -678,7 +701,7 @@
         if (!writableMode) {
             saveBtn.disabled = true;
             saveBtn.title = 'Save not available — read-only source.';
-            fmTextarea.readOnly = true;
+            // fmCM was created with readOnly:!writableMode — nothing more here.
         }
 
         renderToc(tocBody, bodyText, editor);
@@ -788,7 +811,7 @@
         var onChange = debounce(async function () {
             if (currentInstance !== instance) return;
             var body = editor.getMarkdown();
-            var h = await hashContent(assembleContent(fmTextarea.value, body));
+            var h = await hashContent(assembleContent(fmCM.getValue(), body));
             if (currentInstance !== instance) return;
             markDirty(h !== instance.hash);
             renderToc(tocBody, body, editor);
@@ -826,7 +849,7 @@
             while (fmWarn.firstChild) fmWarn.removeChild(fmWarn.firstChild);
             var fid = filenameIdentity(node.name);
             if (!fid || !canSave(node)) { fmWarn.style.display = 'none'; return; }
-            var data = parseFrontMatter('---\n' + fmTextarea.value + '\n---\n').data || {};
+            var data = parseFrontMatter('---\n' + fmCM.getValue() + '\n---\n').data || {};
             var edits = [];
             IDENTITY_FIELDS.forEach(function (f) {
                 if (!(f.fm in data)) return;
@@ -866,13 +889,13 @@
         async function revertIdentityEdits() {
             var fid = filenameIdentity(node.name);
             if (!fid) return;
-            var data = parseFrontMatter('---\n' + fmTextarea.value + '\n---\n').data || {};
+            var data = parseFrontMatter('---\n' + fmCM.getValue() + '\n---\n').data || {};
             for (var k in fid) {
                 if (Object.prototype.hasOwnProperty.call(fid, k)) data[k] = fid[k];
             }
-            fmTextarea.value = stringifyFrontMatter(data);
+            fmCM.setValue(stringifyFrontMatter(data));
             var body = editor.getMarkdown();
-            var h = await hashContent(assembleContent(fmTextarea.value, body));
+            var h = await hashContent(assembleContent(fmCM.getValue(), body));
             if (currentInstance !== instance) return;
             markDirty(h !== instance.hash);
             renderIdentityCue();
@@ -931,18 +954,18 @@
         var onFmChange = debounce(async function () {
             if (currentInstance !== instance) return;
             var body = editor.getMarkdown();
-            var h = await hashContent(assembleContent(fmTextarea.value, body));
+            var h = await hashContent(assembleContent(fmCM.getValue(), body));
             if (currentInstance !== instance) return;
             markDirty(h !== instance.hash);
             renderIdentityCue();
         }, 250);
-        fmTextarea.addEventListener('input', onFmChange);
+        fmCM.on('change', onFmChange);
         renderIdentityCue(); // initial state on load (clean after sync-on-open)
 
         // If sync-on-open corrected the front matter, open the buffer dirty so
         // a save bakes the filename-derived identity in — and say so, since the
         // change is otherwise silent (the values just match the filename now).
-        if (writableMode && fmTextarea.value !== onDiskFM) {
+        if (writableMode && fmCM.getValue() !== onDiskFM) {
             markDirty(true);
             statusEl.textContent = 'Front matter synced to filename — review and save';
         }
@@ -1009,7 +1032,7 @@
         async function save() {
             if (currentInstance !== instance) return;
             if (!instance.dirty || !canSave(node)) return;
-            var content = assembleContent(fmTextarea.value, editor.getMarkdown());
+            var content = assembleContent(fmCM.getValue(), editor.getMarkdown());
             try {
                 statusEl.textContent = 'Saving…';
                 var res = await saveContent(node, content, {