Keep the patch description open while editing

The description collapses past 300px behind a show more toggle, which
also applied to the editor that replaces it. Half of what was being
written could be cut off, with a control offering to hide more of it.

Editing state moves from the revision component to a bindable prop, so
the patch view that owns the collapse can see it.

src/components/Revision.svelte

@@ -103,6 +103,9 @@
     // revision list can be marked, not just the selected one.
     draftRevisionIds?: string[];
     showingRevisionDiff?: boolean;
+    // Whether the description is being edited, so the view around it can keep
+    // it open rather than collapsing what is being typed.
+    editingDescription?: boolean;
     // Review ids (and `STANDALONE_COMMENTS`) whose code comments are
     // hidden from the diff.
     hiddenCommentSources?: string[];
@@ -132,6 +135,7 @@
     draftReviewId,
     draftRevisionIds = [],
     showingRevisionDiff = $bindable(true),
+    editingDescription = $bindable(false),
     hiddenCommentSources = [],
     filesExpanded = $bindable(true),
     onViewChanges,
@@ -1017,8 +1021,6 @@
     }) as Thread[],
   );
 
-  let editingDescription = $state(false);
-
   async function editRevision(
     description: string,
     embeds: Embed[],

src/views/repo/Patch.svelte

@@ -282,8 +282,11 @@
   let descriptionExpanded = $state(false);
   let descriptionEl = $state<HTMLElement>();
   let descriptionOverflows = $state(false);
+  let descriptionEditing = $state(false);
+  // Editing keeps the description open: collapsing it would hide part of what
+  // is being written, and the height is the editor's rather than the text's.
   const descriptionCollapsed = $derived(
-    descriptionOverflows && !descriptionExpanded,
+    descriptionOverflows && !descriptionExpanded && !descriptionEditing,
   );
   $effect(() => {
     const el = descriptionEl;
@@ -1578,9 +1581,10 @@
                       {loadPatch}
                       revision={revisions[0]}
                       {config}
-                      view="description" />
+                      view="description"
+                      bind:editingDescription={descriptionEditing} />
                   </div>
-                  {#if descriptionOverflows}
+                  {#if descriptionOverflows && !descriptionEditing}
                     <div class="patch-description-toggle">
                       <button
                         type="button"