Keep message edits expanded on mobile (#4097)

## Human comments

## What was wrong

Sent-message and queued-message edits reused the follow-up composer’s
compact mobile behavior. After focus left an inline edit, its draft
collapsed to a single row and hid the editing controls. There is no
dedicated issue for this layout bug; nearby queued-editing issues
[#1864](#1864) and
[#2401](#2401) are separate.

## What changed

Added an edit-specific expanded preference to `FollowUpPromptBox` and
opted in the main-thread sent and queued editors plus the embedded-chat
queued editor. The derived expanded state drives the compact layout,
mobile CSS, and height animation. The ordinary follow-up composer keeps
its existing default. Synthetic before/after screenshots are in
`docs/assets/message-editing/`.

## How you verified

- `pnpm exec turbo run test typecheck --filter=@bb/app` — 5,077 tests
passed, 6 skipped; typecheck passed.
- `pnpm exec turbo run lint --filter=@bb/app` — 0 errors.
- At 390 px in the source app, opened sent and queued edits and moved
focus away. Before: `compact: true, expanded: false`. After: `compact:
false, expanded: true`.

> AGENT GENERATED

Author: ymichael

apps/app/src/components/promptbox/FollowUpPromptBox.test.tsx

@@ -917,6 +917,32 @@ describe("FollowUpPromptBox", () => {
     ).toBeNull();
   });
 
+  it("shows the full editor immediately for message edits on mobile", () => {
+    mocks.isCompactViewport = true;
+    vi.useFakeTimers();
+
+    try {
+      const props = createFollowUpPromptBoxProps({ kind: "ready" });
+      render(<FollowUpPromptBox {...props} preferExpanded />);
+      const promptBox = screen.getByTestId("prompt-box");
+      const input = screen.getByRole("textbox", { name: "Follow-up prompt" });
+
+      expect(promptBox.getAttribute("data-compact")).toBe("false");
+      act(() => {
+        input.focus();
+        input.blur();
+        vi.advanceTimersByTime(20);
+      });
+      expect(promptBox.getAttribute("data-compact")).toBe("false");
+      expect(
+        promptBox.closest("[data-follow-up-composer-expanded]"),
+      ).not.toBeNull();
+      expect(screen.queryByText("Ask a follow-up")).toBeNull();
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
   it("collapses a wide composer until the user focuses it again", () => {
     const props = createFollowUpPromptBoxProps({ kind: "ready" });
     props.environmentSummary = <span>Local environment</span>;

apps/app/src/components/promptbox/FollowUpPromptBox.tsx

@@ -167,6 +167,7 @@ export interface FollowUpPromptBoxProps {
   pluginComposerScope?: PluginComposerScope | null;
   textEffects?: readonly ComposerTextEffectSource[];
   collapseResetKey: string | number;
+  preferExpanded?: boolean;
   focusEndKey?: string | number;
   isPrimaryComposer?: boolean;
   showScrollToBottomButton?: boolean;
@@ -238,6 +239,7 @@ function FollowUpPromptBoxWithComposer({
   pluginComposerScope,
   textEffects,
   collapseResetKey,
+  preferExpanded = false,
   focusEndKey,
   isPrimaryComposer = true,
   showScrollToBottomButton = true,
@@ -296,8 +298,10 @@ function FollowUpPromptBoxWithComposer({
   >(null);
   const isWidePromptBoxCollapsed =
     widePromptBoxCollapsedFor === collapseResetKey;
+  const isEditorExpanded =
+    isInteractionExpanded || (preferExpanded && !isWidePromptBoxCollapsed);
   const isPromptBoxCompact =
-    isWidePromptBoxCollapsed || (isCompactViewport && !isInteractionExpanded);
+    isWidePromptBoxCollapsed || (isCompactViewport && !isEditorExpanded);
   const compactConfig = useMemo(
     () =>
       isCompactViewport || isWidePromptBoxCollapsed
@@ -685,7 +689,7 @@ function FollowUpPromptBoxWithComposer({
       ref={composerInteractionRef}
       className="relative z-20"
       data-follow-up-composer=""
-      data-follow-up-composer-expanded={isInteractionExpanded ? "" : undefined}
+      data-follow-up-composer-expanded={isEditorExpanded ? "" : undefined}
       hidden={hasPendingInteraction}
       onBlurCapture={scheduleCollapseAfterFocusLoss}
       onFocusCapture={handleComposerFocus}
@@ -709,7 +713,7 @@ function FollowUpPromptBoxWithComposer({
         focusEndKey={focusEndKey}
         placeholder={composer.promptPlaceholder}
         containerCompactPlaceholder={composer.compactPromptPlaceholder}
-        heightAnimationKey={isInteractionExpanded ? "expanded" : "compact"}
+        heightAnimationKey={isEditorExpanded ? "expanded" : "compact"}
         mentionMenuPlacement="top"
         submission={{
           label: composer.submitLabel,

apps/app/src/components/thread/embedded-chat/EmbeddedThreadChat.tsx

@@ -1011,6 +1011,7 @@ function EmbeddedThreadChatWithComposer({
           typeahead={typeaheadConfig}
           promptActions={promptActions}
           collapseResetKey={`${surfaceKey}:queued-message:${inlineEditingQueuedMessage.queuedMessageId}`}
+          preferExpanded
           focusEndKey={`${inlineEditingQueuedMessage.editSessionId}:${inlineComposerFocusNonce}`}
           isPrimaryComposer={false}
           showScrollToBottomButton={false}

apps/app/src/views/thread-detail/ThreadDetailPromptArea.tsx

@@ -287,6 +287,7 @@ function buildInlineDraftComposer(options: InlineDraftComposerOptions) {
       typeahead={options.typeahead}
       promptActions={options.promptActions}
       collapseResetKey={options.collapseResetKey}
+      preferExpanded
       focusEndKey={`${options.focusSessionKey}:${options.editFocusNonce}`}
       isPrimaryComposer={false}
       showScrollToBottomButton={false}