fix(editor-v2): reserve editor slot to stop layout shift on mount

The previous flicker fix hid the textarea via CSS but display: none
removes it from layout entirely — so the page rendered with zero
height where the editor would go, then cm6 mounted and pushed the
surrounding form down by its full height (CLS).

Wrap each editor textarea in <div class="editor-mount" style="min-height: …rem">
so the slot is reserved before cm6 mounts. The wrapper is a flex
column with cm6 as flex: 1 so cm6 fills the reserved space rather
than collapsing to content-height with a gap below (the seeded
blueprint has 2 chars of content; without flex the editor would
shrink to one line).

Min-heights calibrated to rows × ~1.25rem + ~1.5rem chrome:
- config (rows=8)  → 12rem
- files (rows=14)  → 19rem
- script (rows=20) → 27rem

.cm-editor's own min-height: 8em rule removed — the wrapper is the
floor now, and the inner cm6 stretches to fill via flex.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

l4d2web/l4d2web/static/css/editor.css

@@ -9,10 +9,21 @@
  * in _editor_assets.html un-hides for JS-disabled users. */
 textarea[data-editor-language] { display: none; }
 
+/* Wrapper that reserves layout space before cm6 mounts, sized by the
+ * inline `min-height` style each call site sets to match the original
+ * textarea's `rows`. cm6 is a flex child that fills the reserved space
+ * (otherwise short content would render cm6 small with a gap below). */
+.editor-mount {
+  display: flex;
+  flex-direction: column;
+}
+.editor-mount > .cm-editor {
+  flex: 1;
+}
+
 .cm-editor {
   border: var(--line);
   border-radius: var(--radius-s);
-  min-height: 8em;
 }
 
 .cm-editor.cm-focused {

l4d2web/l4d2web/templates/blueprint_detail.html

@@ -51,7 +51,7 @@
         <pre class="config-preview" aria-label="Auto-loaded overlay configs">{% for o in exposed %}exec {{ o.name }}.cfg
 {% endfor %}</pre>
         {% endif %}
-        <textarea name="config" rows="8" spellcheck="false" data-editor-language="srccfg">{{ config_lines | join('\n') }}</textarea>
+        <div class="editor-mount" style="min-height: 12rem"><textarea name="config" rows="8" spellcheck="false" data-editor-language="srccfg">{{ config_lines | join('\n') }}</textarea></div>
       </div>
     </label>
     <button type="submit">Save blueprint</button>

l4d2web/l4d2web/templates/overlay_detail.html

@@ -24,7 +24,7 @@
   <form method="post" action="/overlays/{{ overlay.id }}/script" class="stack">
     <input type="hidden" name="csrf_token" value="{{ session.get('csrf_token', '') }}">
     <label>Bash script
-      <textarea name="script" rows="20" spellcheck="false" data-editor-language="bash">{{ overlay.script or "" }}</textarea>
+      <div class="editor-mount" style="min-height: 27rem"><textarea name="script" rows="20" spellcheck="false" data-editor-language="bash">{{ overlay.script or "" }}</textarea></div>
     </label>
     <p class="muted">Runs sandboxed against the overlay directory mounted at <code>/overlay</code>.</p>
     {% if not latest_build_is_running %}
@@ -186,7 +186,7 @@
       </label>
       <label class="files-editor-field">
         <span class="files-field-label">Content</span>
-        <textarea class="files-editor-content" rows="14" spellcheck="false" data-editor-language="auto"></textarea>
+        <div class="editor-mount" style="min-height: 19rem"><textarea class="files-editor-content" rows="14" spellcheck="false" data-editor-language="auto"></textarea></div>
       </label>
       <div class="files-editor-meta muted">
         <span class="files-editor-byte-count">UTF-8 · 0 bytes</span>