diff --git a/components/mail/__tests__/mail-app-list-ux.test.tsx b/components/mail/__tests__/mail-app-list-ux.test.tsx
index 7f11c1bf9..b209cea91 100644
--- a/components/mail/__tests__/mail-app-list-ux.test.tsx
+++ b/components/mail/__tests__/mail-app-list-ux.test.tsx
@@ -37,8 +37,8 @@ vi.mock('@/components/ui/welcome-banner', () => ({
   useWelcomeBannerDue: () => false,
 }));
 vi.mock('@/components/email/email-viewer', () => ({
-  EmailViewer: ({ emptyPaneContent, emptyPaneFooter }: { emptyPaneContent?: React.ReactNode; emptyPaneFooter?: React.ReactNode }) => (
-    <div data-testid="viewer">{emptyPaneContent ?? <p>no conversation selected</p>}{emptyPaneFooter}</div>
+  EmailViewer: ({ email, emptyPaneContent, emptyPaneFooter }: { email?: { id: string } | null; emptyPaneContent?: React.ReactNode; emptyPaneFooter?: React.ReactNode }) => (
+    <div data-testid="viewer">{email ? <p>reading {email.id}</p> : <>{emptyPaneContent ?? <p>no conversation selected</p>}{emptyPaneFooter}</>}</div>
   ),
 }));
 vi.mock('@/components/email/email-list', () => ({
@@ -94,7 +94,7 @@ beforeEach(() => {
     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,
+    selectedEmailKeys: new Set(), unreadSection: null, listRows: 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),
@@ -138,6 +138,35 @@ describe('the reading pane for a selection', () => {
     expect(screen.getByTestId('viewer').contains(listProps.host!)).toBe(true);
   });
 
+  it('replaces the open message once other rows are ticked too (mail-p2-3)', () => {
+    useEmailStore.setState({ selectedEmail: message, selectedEmailKeys: new Set(['m1', 'm2']) } as never);
+    render(<MailApp />);
+    expect(screen.queryByText('reading m1')).toBeNull();
+    expect(listProps.host).toBeInstanceOf(HTMLElement);
+    expect(screen.getByTestId('viewer').contains(listProps.host!)).toBe(true);
+  });
+
+  it('replaces the open message when another single row is ticked', () => {
+    useEmailStore.setState({ selectedEmail: message, selectedEmailKeys: new Set(['m2']) } as never);
+    render(<MailApp />);
+    expect(screen.queryByText('reading m1')).toBeNull();
+  });
+
+  it('keeps the open message when only its own row is ticked', () => {
+    useEmailStore.setState({ selectedEmail: message, selectedEmailKeys: new Set(['m1']) } as never);
+    render(<MailApp />);
+    expect(screen.getByText('reading m1')).toBeTruthy();
+  });
+
+  it('keeps the open message when its conversation row is ticked', () => {
+    useEmailStore.setState({
+      selectedEmail: message, selectedEmailKeys: new Set(['m1', 'm1-reply']),
+      listRows: [{ key: 'm1', members: ['m1', 'm1-reply'] }, { key: 'm2', members: ['m2'] }],
+    } as never);
+    render(<MailApp />);
+    expect(screen.getByText('reading m1')).toBeTruthy();
+  });
+
   it('says nothing is selected without a selection', () => {
     render(<MailApp />);
     expect(screen.getByText('no conversation selected')).toBeTruthy();
diff --git a/components/mail/mail-app.tsx b/components/mail/mail-app.tsx
index bdc888cfc..4853163de 100644
--- a/components/mail/mail-app.tsx
+++ b/components/mail/mail-app.tsx
@@ -4927,8 +4927,23 @@ export function MailApp({ linkSegments: routeSegments }: MailAppProps = {}) {
   const dockedReply = dockCandidate && dockedSessionRef.current === composerSessionId;
   const composerHost = floatingComposer ? 'window' : mvInlineReply ? 'inline' : dockedReply ? 'dock' : 'pane';
   composerHostRef.current = composerHost;
+  // Rows ticked beyond the open message take the reading pane (mail-p2-3):
+  // the selection bar and the keys act on the ticked rows, so a reader
+  // toolbar acting on the one open message would be a second, different set.
+  // Unticking brings the message back. Mountain View keeps the message while
+  // rows are ticked (#1231); the focus layout can't tick rows while reading.
+  const openRowKeys = (() => {
+    if (!selectedEmail) return null;
+    const key = emailKeyFor(selectedEmail);
+    // A conversation row ticks all its messages: the open one's whole row.
+    const row = useEmailStore.getState().listRows?.find((r) => r.key === key || r.members.includes(key));
+    return new Set(row ? [key, ...row.members] : [key]);
+  })();
+  const selectionTakesReader = !isMobile && !isFocusedMailLayout && !mvShell && !isScheduledView
+    && selectedEmailKeys.size > 0
+    && !(openRowKeys && [...selectedEmailKeys].every((key) => openRowKeys.has(key)));
   const hasViewerContent = (showComposer && !floatingComposer) || Boolean(conversationThread)
-    || Boolean(selectedEmail && !(mvShell && cursorOnly)) || deepLinkPane !== null;
+    || Boolean(selectedEmail && !(mvShell && cursorOnly) && !selectionTakesReader) || deepLinkPane !== null;
   // A linked message on a phone opens over the list, as one picked there does.
   const mobileView = deepLinkPane !== null && isMobile ? 'viewer' : activeView;
   const shouldCollapseListPane = !isMobile && isFocusedMailLayout && hasViewerContent;
@@ -6106,9 +6121,9 @@ export function MailApp({ linkSegments: routeSegments }: MailAppProps = {}) {
               <>
                 <ErrorBoundary fallback={EmailViewerErrorFallback}>
                   <EmailViewer
-                    email={selectedEmail}
+                    email={selectionTakesReader ? null : selectedEmail}
                     blobSource={resolveEmailBlobContext(selectedEmail, client)}
-                    isLoading={isLoadingEmail || deepLinkPane !== null}
+                    isLoading={!selectionTakesReader && (isLoadingEmail || deepLinkPane !== null)}
                     onReply={handleReply}
                     onReplyAll={handleReplyAll}
                     onForward={handleForward}