diff --git a/components/mail/__tests__/mail-app-composer-replace.test.tsx b/components/mail/__tests__/mail-app-composer-replace.test.tsx
new file mode 100644
index 000000000..f56066c13
--- /dev/null
+++ b/components/mail/__tests__/mail-app-composer-replace.test.tsx
@@ -0,0 +1,174 @@
+import React from 'react';
+import { act, fireEvent, render, screen, waitFor } from '@testing-library/react';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import type { Mailbox } from '@/lib/jmap/types';
+import { useAuthStore } from '@/stores/auth-store';
+import { useEmailStore } from '@/stores/email-store';
+import { useSettingsStore } from '@/stores/settings-store';
+import { usePluginStore } from '@/stores/plugin-store';
+import { useThemeStore } from '@/stores/theme-store';
+import { useToastStore } from '@/stores/toast-store';
+import { DEFAULT_SEARCH_FILTERS } from '@/lib/jmap/search-utils';
+
+// One composer at a time (compose-p2-1, compose-p2-2). Another one opening
+// while a composer is on screen closes that one the way its X does (save,
+// "Draft saved"), then opens. It never crashes and never drops the text.
+
+const device = vi.hoisted(() => ({ current: { isMobile: false, isTablet: false, isDesktop: true } }));
+// What the stub composer's close does: 'saves' closes and lets the next one
+// open, 'fails' keeps the composer (a save the server refused).
+const stub = vi.hoisted(() => ({
+  close: 'none' as 'none' | 'saves' | 'fails',
+  closes: [] as string[],
+  mounts: [] as string[],
+}));
+
+vi.mock('@/components/layout/mv-top-bar', () => ({ MvTopBar: () => null }));
+vi.mock('@/components/layout/sidebar', () => ({
+  Sidebar: ({ onCompose }: { onCompose: () => void }) => <button onClick={onCompose}>stub compose</button>,
+}));
+// As the real composer: it hands its state (its own mode, untouched) to the
+// host as it unmounts without an explicit close, and offers its close.
+vi.mock('@/components/email/email-composer', async () => {
+  const { useEffect, useRef } = await import('react');
+  return {
+    EmailComposer: ({ mode, onSaveState, onClose, requestCloseRef }: {
+      mode: string;
+      onSaveState: (data: Record<string, unknown>) => void;
+      onClose: () => void;
+      requestCloseRef?: { current: unknown };
+    }) => {
+      const closed = useRef(false);
+      useEffect(() => {
+        stub.mounts.push(mode);
+        // The old host moved the composer between dock and pane for ever
+        // (React error 185 in the browser); stop it here instead of hanging.
+        if (stub.mounts.length > 40) throw new Error('composer remount loop');
+        if (requestCloseRef && stub.close !== 'none') {
+          requestCloseRef.current = (afterClose?: () => void) => {
+            stub.closes.push(mode);
+            if (stub.close === 'fails') return;
+            closed.current = true;
+            onClose();
+            afterClose?.();
+          };
+        }
+        return () => {
+          if (requestCloseRef) requestCloseRef.current = null;
+          if (closed.current) return;
+          onSaveState({ to: '', cc: '', bcc: '', subject: '', body: '', mode, draftId: null, unedited: true });
+        };
+        // eslint-disable-next-line react-hooks/exhaustive-deps
+      }, []);
+      return <div data-testid="stub-composer" data-mode={mode}>stub composer</div>;
+    },
+  };
+});
+vi.mock('@/components/search/mv-search-form', () => ({
+  MvSearchForm: ({ onSearch }: { onSearch: () => void }) => <button onClick={onSearch}>stub filter search</button>,
+}));
+vi.mock('@/components/email/email-viewer', () => ({
+  EmailViewer: ({ onReply, onReplyAll, onForward }: { onReply: () => void; onReplyAll: () => void; onForward: () => void }) => (
+    <div data-testid="viewer">
+      message viewer
+      <button onClick={() => onReply()}>stub reply</button>
+      <button onClick={() => onReplyAll()}>stub reply all</button>
+      <button onClick={() => onForward()}>stub forward</button>
+    </div>
+  ),
+}));
+vi.mock('@/components/layout/navigation-rail', () => ({ NavigationRail: () => null }));
+vi.mock('@/components/layout/calendar-event-notification-toaster', () => ({ CalendarEventNotificationToaster: () => null }));
+vi.mock('@/components/layout/share-notification-toaster', () => ({ ShareNotificationToaster: () => null }));
+vi.mock('@/components/layout/mobile-header', () => ({ MobileHeader: () => null }));
+vi.mock('@/components/layout/mv-side-panel', () => ({ MvSidePanel: () => null }));
+vi.mock('@/components/email/message-list-tabs', () => ({ useMessageListPanelProps: (id: string) => ({ id }), MessageListTabs: () => null }));
+vi.mock('@/components/search/search-box', () => ({ SearchBox: () => null }));
+vi.mock('@/components/tour/tour-provider', () => ({ useTour: () => ({ activeTour: null, registerStep: vi.fn() }) }));
+vi.mock('@/components/plugins/plugin-slot', () => ({ PluginSlot: () => null }));
+vi.mock('@/hooks/use-plugin-slot-offers', () => ({ usePluginSlotOffers: () => [] }));
+vi.mock('@/hooks/use-config', () => ({ useConfig: () => ({ appName: 'Mail' }) }));
+vi.mock('@/hooks/use-identity-sync', () => ({ useIdentitySync: () => {} }));
+vi.mock('@/hooks/use-pro-multi-account-identities', () => ({ useProMultiAccountIdentities: () => [] }));
+vi.mock('@/hooks/use-pro-multi-account-mailboxes', () => ({ useProMultiAccountMailboxes: () => {} }));
+vi.mock('@/hooks/use-wopi-status', () => ({ useWopiStatus: () => null, canWopiOpen: () => false }));
+vi.mock('@/hooks/use-browser-navigation', () => ({ useBrowserNavigation: () => {} }));
+vi.mock('@/hooks/use-media-query', () => ({
+  useDeviceDetection: () => device.current,
+  useMediaQuery: () => false,
+  useIsDesktop: () => true,
+}));
+vi.mock('@/lib/push-bindings', () => ({ reconcilePushBindings: () => {}, releasePushBindings: () => {} }));
+vi.mock('@/components/email/email-list', () => ({
+  EmailList: () => null,
+}));
+
+const { MailApp } = await import('../mail-app');
+
+const inbox = { id: 'inbox', name: 'Inbox', role: 'inbox', totalEmails: 0, unreadEmails: 0 } as Mailbox;
+const MESSAGE = {
+  id: 'm1', threadId: 't1', mailboxIds: { inbox: true }, keywords: { $seen: true },
+  from: [{ name: 'Ann', email: 'ann@example.org' }], to: [{ email: 'a@example.org' }],
+  subject: 'Plans', receivedAt: '2026-11-01T10:00:00Z', preview: 'Hi', size: 10,
+  hasAttachment: false, textBody: [], htmlBody: [], bodyValues: {},
+} as never;
+
+beforeEach(() => {
+  device.current = { isMobile: false, isTablet: false, isDesktop: true };
+  stub.close = 'none';
+  stub.closes = [];
+  stub.mounts = [];
+  vi.stubGlobal('fetch', vi.fn().mockRejectedValue(new Error('Unexpected network request')));
+  vi.stubGlobal('ResizeObserver', class { observe() {} unobserve() {} disconnect() {} });
+  usePluginStore.setState({ initializePlugins: vi.fn().mockResolvedValue(undefined) });
+  useThemeStore.setState({ syncServerThemes: vi.fn().mockResolvedValue(undefined) });
+  const client = {
+    getAccountId: () => 'acc', getUsername: () => 'a@example.org',
+    getSieveAccountId: () => null, hasDelayedSend: () => false, getSession: () => null,
+    getEmail: vi.fn().mockResolvedValue(null),
+  };
+  useAuthStore.setState({
+    isAuthenticated: true, isLoading: false, client: client as never, activeAccountId: 'login-a', identities: [],
+    getClientForAccount: () => client as never,
+    getAllConnectedClients: () => new Map([['login-a', client]]) as never,
+  });
+  useSettingsStore.setState({ enableUnifiedMailbox: false, interfaceLayout: 'default', mailLayout: 'split' } as never);
+  useEmailStore.setState({
+    emails: [MESSAGE], selectedEmail: MESSAGE, selectedMailbox: 'inbox', selectedKeyword: null, mailboxes: [inbox],
+    viewingAccountId: null, accountMailboxes: {}, isUnifiedView: false, unifiedRole: null,
+    isScheduledView: false, isLoadingEmail: false, isLoading: false, searchQuery: '',
+    searchFilters: { ...DEFAULT_SEARCH_FILTERS }, searchMailboxId: '', isAdvancedSearchOpen: false,
+    selectedEmailKeys: new Set(), unreadSection: null,
+    fetchMailboxes: vi.fn().mockResolvedValue(undefined), fetchEmails: vi.fn().mockResolvedValue(undefined),
+    fetchQuota: vi.fn().mockResolvedValue(undefined), fetchTagCounts: vi.fn().mockResolvedValue(undefined),
+    refreshScheduledMetadata: vi.fn().mockResolvedValue(undefined),
+  } as never);
+  useToastStore.getState().clearToasts();
+});
+afterEach(() => {
+  vi.restoreAllMocks();
+  vi.unstubAllGlobals();
+  useToastStore.getState().clearToasts();
+});
+
+const composerMode = () => screen.getByTestId('stub-composer').getAttribute('data-mode');
+
+describe('Compose while a reply is docked (compose-p2-1)', () => {
+  for (const action of ['stub reply', 'stub forward']) {
+    it(`opens a new message in the pane after ${action}`, async () => {
+      render(<MailApp />);
+      fireEvent.click(await screen.findByText(action));
+      const reply = await screen.findByTestId('stub-composer');
+      expect(reply.closest('[data-reply-dock]')).not.toBeNull();
+
+      fireEvent.click(screen.getByText('stub compose'));
+      await waitFor(() => expect(composerMode()).toBe('compose'));
+      // The new message takes the pane, not the dock, and stays put.
+      expect(screen.getByTestId('stub-composer').closest('[data-reply-dock]')).toBeNull();
+      const mounted = stub.mounts.length;
+      await act(async () => { await new Promise((r) => setTimeout(r, 20)); });
+      expect(stub.mounts.length).toBe(mounted);
+      expect(composerMode()).toBe('compose');
+    });
+  }
+});
diff --git a/components/mail/mail-app.tsx b/components/mail/mail-app.tsx
index bdc888cfc..1ea598b03 100644
--- a/components/mail/mail-app.tsx
+++ b/components/mail/mail-app.tsx
@@ -304,7 +304,7 @@ export function MailApp({ linkSegments: routeSegments }: MailAppProps = {}) {
   composerSessionRef.current = composerSessionId;
   const composerHostRef = useRef('pane');
   // The compose session docked under its message (dockedReply).
-  const dockedSessionRef = useRef<number | null>(null);
+  const [dockedSession, setDockedSession] = useState<number | null>(null);
   const showComposerRef = useRef(showComposer);
   showComposerRef.current = showComposer;
   // Fields a plugin filled in through onBeforeCompose for the composer about
@@ -4923,8 +4923,9 @@ export function MailApp({ linkSegments: routeSegments }: MailAppProps = {}) {
   // anew when the message goes away (another folder opened meanwhile).
   const dockCandidate = !mvShell && !isMobile && showComposer && !isEmbedded && !floatingComposer
     && (pendingDraft?.mode ?? composerMode) !== 'compose';
-  if (dockCandidate && selectedEmail && !selectedEmail.keywords?.$draft) dockedSessionRef.current = composerSessionId;
-  const dockedReply = dockCandidate && dockedSessionRef.current === composerSessionId;
+  const dockOnOpen = dockCandidate && Boolean(selectedEmail) && !selectedEmail?.keywords?.$draft;
+  if (dockOnOpen && dockedSession !== composerSessionId) setDockedSession(composerSessionId);
+  const dockedReply = dockCandidate && (dockOnOpen || dockedSession === composerSessionId);
   const composerHost = floatingComposer ? 'window' : mvInlineReply ? 'inline' : dockedReply ? 'dock' : 'pane';
   composerHostRef.current = composerHost;
   const hasViewerContent = (showComposer && !floatingComposer) || Boolean(conversationThread)
@@ -5275,6 +5276,12 @@ export function MailApp({ linkSegments: routeSegments }: MailAppProps = {}) {
         sessionKey={`inline-${composerSessionId}`}
         sourceEmailId={answeredEmailId}
         onSaveState={(data) => {
+          // A composer whose session another one has replaced: its state
+          // belongs to no composer on screen. Taken as the new session's,
+          // it turned a new message into the old reply and moved it between
+          // the dock and the pane for ever (React error 185, compose-p2-1).
+          // Whatever opens a composer closes the old one first, which saves it.
+          if (composerSessionRef.current !== composerSessionId) return;
           setComposerPrefill(null);
           if (suppressComposerStateSaveSessionRef.current === composerSessionId) {
             suppressComposerStateSaveSessionRef.current = null;