diff --git a/components/email/__tests__/reading-mode-toggle.test.tsx b/components/email/__tests__/reading-mode-toggle.test.tsx
new file mode 100644
index 000000000..091740c7b
--- /dev/null
+++ b/components/email/__tests__/reading-mode-toggle.test.tsx
@@ -0,0 +1,102 @@
+import { render, screen } from '@testing-library/react';
+import { describe, it, expect, vi } from 'vitest';
+import type { Email, Mailbox } from '@/lib/jmap/types';
+
+// #964: the reading-mode (Sun/Moon) toggle used to mount only once the body
+// had loaded and turned out to be HTML. That late mount changed the toolbar
+// width and re-ran the overflow calculation, so the other buttons jumped.
+// The toggle must be in the DOM from the start, merely disabled.
+
+vi.mock('@/hooks/use-media-query', () => ({
+  useDeviceDetection: () => ({ isMobile: false, isTablet: false, isDesktop: true }),
+  useMediaQuery: () => false,
+}));
+
+vi.mock('@/components/tour/tour-provider', () => ({
+  useTour: () => ({ activeTour: null, startTour: vi.fn(), endTour: vi.fn(), registerStep: vi.fn() }),
+}));
+
+vi.mock('@/components/plugins/plugin-slot', () => ({
+  PluginSlot: () => null,
+}));
+
+vi.mock('@/hooks/use-plugin-slot-offers', () => ({
+  usePluginSlotOffers: () => [],
+}));
+
+class NoopResizeObserver {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+}
+vi.stubGlobal('ResizeObserver', NoopResizeObserver);
+
+const { EmailViewer } = await import('../email-viewer');
+
+const mailboxes: Mailbox[] = [
+  { id: 'inbox', name: 'Inbox', role: 'inbox', parentId: null, myRights: { mayAddItems: true } },
+] as unknown as Mailbox[];
+
+function makeEmail(overrides: Partial<Email> = {}): Email {
+  return {
+    id: 'e1',
+    blobId: 'b1',
+    threadId: 't1',
+    mailboxIds: { inbox: true },
+    keywords: { $seen: true },
+    from: [{ name: 'Alice', email: 'alice@example.com' }],
+    to: [{ name: 'Bob', email: 'bob@example.com' }],
+    subject: 'Hello',
+    receivedAt: '2026-01-01T00:00:00Z',
+    preview: 'Hello',
+    hasAttachment: false,
+    size: 100,
+    textBody: [],
+    htmlBody: [],
+    attachments: [],
+    bodyValues: {},
+    ...overrides,
+  } as unknown as Email;
+}
+
+function findToggle(): HTMLElement | null {
+  return (
+    screen.queryByTitle('View in dark mode') ?? screen.queryByTitle('View in light mode')
+  );
+}
+
+describe('reading-mode toggle stays mounted (#964)', () => {
+  it('is in the DOM while the body is still loading, disabled', () => {
+    render(
+      <EmailViewer
+        email={makeEmail({ htmlBody: [{ partId: '1', type: 'text/html' }] } as never)}
+        isLoading
+        mailboxes={mailboxes}
+        selectedMailbox="inbox"
+        currentMailboxRole="inbox"
+      />,
+    );
+
+    const toggle = findToggle();
+    expect(toggle).not.toBeNull();
+    expect(toggle).toBeDisabled();
+  });
+
+  it('is enabled once the HTML body is available', () => {
+    render(
+      <EmailViewer
+        email={makeEmail({
+          htmlBody: [{ partId: '1', type: 'text/html' }],
+          bodyValues: { '1': { value: '<p>Hi</p>', isTruncated: false } },
+        } as never)}
+        mailboxes={mailboxes}
+        selectedMailbox="inbox"
+        currentMailboxRole="inbox"
+      />,
+    );
+
+    const toggle = findToggle();
+    expect(toggle).not.toBeNull();
+    expect(toggle).not.toBeDisabled();
+  });
+});
diff --git a/components/email/email-viewer.tsx b/components/email/email-viewer.tsx
index 8ee35b017..0182c761f 100644
--- a/components/email/email-viewer.tsx
+++ b/components/email/email-viewer.tsx
@@ -3218,8 +3218,10 @@ export function EmailViewer({
           <Code className="w-4 h-4" />
         </Button>
 
-        {/* Dark/light mode toggle for HTML emails */}
-        {effectiveEmailContent.isHtml && (
+        {/* Dark/light mode toggle for HTML emails. Always mounted: `isHtml`
+            flips from false to true once the body arrives, and mounting the
+            button then changes the toolbar width and re-runs the overflow
+            calculation, so the other buttons jump. Disable it instead. (#964) */}
         <Button
           variant="ghost"
           size="sm"
@@ -3228,10 +3230,11 @@ export function EmailViewer({
           data-overflow-priority="11"
           className="hidden sm:inline-flex h-8 gap-1.5"
           title={isDark ? 'View in light mode' : 'View in dark mode'}
+          disabled={!effectiveEmailContent.isHtml}
+          aria-disabled={!effectiveEmailContent.isHtml}
         >
           {isDark ? <Sun className="w-4 h-4" /> : <Moon className="w-4 h-4" />}
         </Button>
-        )}
 
         {/* Fullscreen toggle - hidden on mobile (already fullscreen there).
             Never overflows into the More menu: in fullscreen this button is