fix: follow the typed subject in a Pro compose tab's title

components/email/email-composer.tsx

@@ -348,6 +348,8 @@ interface EmailComposerProps {
    */
   onDiscardDraft?: (draftId: string, draft?: { accountId?: string; localAccountId?: string }) => void;
   onSaveState?: (data: ComposerDraftData) => void;
+  /** The subject as typed, for a host that shows it (a Pro compose tab's title). */
+  onSubjectChange?: (subject: string) => void;
   className?: string;
   initialDraftText?: string;
   initialData?: ComposerDraftData | null;
@@ -494,6 +496,7 @@ export function EmailComposer({
   requestCloseRef,
   onDiscardDraft,
   onSaveState,
+  onSubjectChange,
   className,
   initialDraftText: initialDraftTextProp,
   initialData: initialDataProp,
@@ -1828,6 +1831,10 @@ export function EmailComposer({
   // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [effectiveSessionKey]);
 
+  useEffect(() => {
+    onSubjectChange?.(subject);
+  }, [onSubjectChange, subject]);
+
   // Auto-save state on unmount (when user navigates away without explicitly closing)
   useEffect(() => {
     return () => {

components/pro/__tests__/pro-compose-tab-title.test.tsx

@@ -5,7 +5,13 @@ import React from 'react';
 // The mail tab hands every compose tab the selected message as replyTo, a new
 // message too. A new message was titled after whatever message was open.
 
-vi.mock('@/components/email/email-composer', () => ({ EmailComposer: () => React.createElement('div') }));
+const composer = vi.hoisted(() => ({ onSubjectChange: null as ((s: string) => void) | null }));
+vi.mock('@/components/email/email-composer', () => ({
+  EmailComposer: (props: { onSubjectChange: (s: string) => void }) => {
+    composer.onSubjectChange = props.onSubjectChange;
+    return React.createElement('div');
+  },
+}));
 vi.mock('@/components/error', () => ({
   ErrorBoundary: ({ children }: { children: React.ReactNode }) => React.createElement(React.Fragment, null, children),
   ComposerErrorFallback: () => null,
@@ -35,20 +41,23 @@ import { ProComposeTabBody } from '../pro-compose-tab-body';
 const selected = { subject: 'Invoice August' };
 
 describe('Pro compose tab title', () => {
-  beforeEach(() => updateTabTitle.mockClear());
-
-  it('stays the new-message title with a message selected in the mail tab', () => {
-    render(<ProComposeTabBody tabId="t" data={{ sessionId: 1, mode: 'compose', title: 'New message', replyTo: selected }} />);
-    expect(updateTabTitle).toHaveBeenLastCalledWith('t', 'New message');
+  beforeEach(() => {
+    updateTabTitle.mockClear();
+    composer.onSubjectChange = null;
   });
 
-  it('keeps the prefix the opener gave a reply', () => {
-    render(<ProComposeTabBody tabId="t" data={{ sessionId: 1, mode: 'reply', title: 'Re: Invoice August', replyTo: selected }} />);
-    expect(updateTabTitle).toHaveBeenLastCalledWith('t', 'Re: Invoice August');
+  it('does not take the title of the message selected in the mail tab', () => {
+    render(<ProComposeTabBody tabId="t" data={{ sessionId: 1, mode: 'compose', title: 'email_composer.new_message', replyTo: selected }} />);
+    composer.onSubjectChange!('');
+    expect(updateTabTitle).toHaveBeenLastCalledWith('t', 'email_composer.new_message');
+    expect(updateTabTitle).not.toHaveBeenCalledWith('t', 'Invoice August');
   });
 
-  it('follows the subject of a reopened draft', () => {
-    render(<ProComposeTabBody tabId="t" data={{ sessionId: 1, mode: 'compose', title: 'x', initialData: { subject: 'Lunch?' } as never }} />);
+  it('follows the subject as it is typed, and falls back when it is emptied', () => {
+    render(<ProComposeTabBody tabId="t" data={{ sessionId: 1, mode: 'compose', title: 'email_composer.new_message' }} />);
+    composer.onSubjectChange!('Lunch?');
     expect(updateTabTitle).toHaveBeenLastCalledWith('t', 'Lunch?');
+    composer.onSubjectChange!('   ');
+    expect(updateTabTitle).toHaveBeenLastCalledWith('t', 'email_composer.new_message');
   });
 });

components/pro/pro-compose-tab-body.tsx

@@ -195,21 +195,15 @@ export function ProComposeTabBody({ tabId, data }: ProComposeTabBodyProps) {
 
   const handleSaveState = useCallback((state: ComposerDraftData) => {
     updateComposeDraft(tabIdRef.current, state);
-    // Keep the tab title in sync with the working subject.
-    const subject = state.subject?.trim() || t('email_composer.new_message');
-    updateTabTitle(tabIdRef.current, subject);
-  }, [updateComposeDraft, updateTabTitle, t]);
+  }, [updateComposeDraft]);
 
-  // On first mount, ensure the tab title matches whatever subject we were
-  // initialised with. The opener's title already says "Re: …" or "Fwd: …" for
-  // a reply or forward. replyTo is no source for it: the mail tab fills that
-  // in from the selected message for a new message too.
-  useEffect(() => {
-    const title = data.initialData?.subject?.trim() || data.title || t('email_composer.new_message');
-    updateTabTitle(tabIdRef.current, title);
-    // Run once on mount only.
-    // eslint-disable-next-line react-hooks/exhaustive-deps
-  }, []);
+  // The tab's title is the subject as it is typed, so several open compose
+  // tabs can be told apart. replyTo is no source for it: the mail tab fills
+  // that in from the selected message for a new message too.
+  const newMessageTitle = t('email_composer.new_message');
+  const handleSubjectChange = useCallback((subject: string) => {
+    updateTabTitle(tabIdRef.current, subject.trim() || newMessageTitle);
+  }, [updateTabTitle, newMessageTitle]);
 
   // Mirrors the standard shell (see the same prop in `components/mail/mail-app.tsx`).
   // Without it `mode === 'compose'` resolves `findComposeIdentityId(identities,
@@ -252,6 +246,7 @@ export function ProComposeTabBody({ tabId, data }: ProComposeTabBodyProps) {
           requestCloseRef={requestCloseRef}
           onDiscardDraft={handleDiscardDraft}
           onSaveState={handleSaveState}
+          onSubjectChange={handleSubjectChange}
           className="flex-1"
         />
       </ErrorBoundary>