diff --git a/components/email/__tests__/composer-plugin-surfaces.test.tsx b/components/email/__tests__/composer-plugin-surfaces.test.tsx
index c21f2494e..f236cba0b 100644
--- a/components/email/__tests__/composer-plugin-surfaces.test.tsx
+++ b/components/email/__tests__/composer-plugin-surfaces.test.tsx
@@ -663,6 +663,21 @@ describe('composer-sidebar and composer-sidebar-right', () => {
     expect(panel('right').dataset.composerSidebarMode).toBe('folded');
   });
 
+  it('answer no request on a phone, where the panels are not shown', () => {
+    const width = window.innerWidth;
+    window.innerWidth = 390;
+    try {
+      stubWidth(390);
+      offerSlots('p1', ['composer-sidebar-right']);
+      render(<EmailComposer initialData={DRAFT} onClose={vi.fn()} />);
+      let open: boolean | null = true;
+      act(() => { open = requestPanel('p1', 'composer-sidebar-right', 'open', undefined); });
+      expect(open).toBeNull();
+    } finally {
+      window.innerWidth = width;
+    }
+  });
+
   it('open a folded panel over the composer when there is no room beside it', () => {
     stubWidth(600);
     offerSlots('p1', ['composer-sidebar-right']);
diff --git a/components/email/composer-plugin-sidebars.tsx b/components/email/composer-plugin-sidebars.tsx
index 4f5eacabe..25360fd45 100644
--- a/components/email/composer-plugin-sidebars.tsx
+++ b/components/email/composer-plugin-sidebars.tsx
@@ -22,6 +22,8 @@ export const MIN_COMPOSER_SIDEBAR_WIDTH = 220;
 export const MAX_COMPOSER_SIDEBAR_WIDTH = 480;
 /** Width of a folded panel (the strip with its toggle). */
 export const COMPOSER_SIDEBAR_STRIP = 37;
+/** Below this window width the panels are not shown at all (the `md` breakpoint). */
+export const COMPOSER_SIDEBAR_MIN_WINDOW = 768;
 /**
  * Narrowest the composer itself may get next to open panels: below this its
  * bottom bar (Attach, Templates, Options, Discard, Send) runs into itself.
@@ -170,6 +172,9 @@ export function useComposerSidebars(root: HTMLElement | null, composerId: string
     const state = latest.current;
     if (request.composerId && request.composerId !== state.composerId) return null;
     if (!(side === "left" ? state.hasLeft : state.hasRight)) return null;
+    // On a phone the panels are not on screen (hidden below md): the plugin
+    // then falls back to a dialog.
+    if (typeof window !== "undefined" && window.innerWidth < COMPOSER_SIDEBAR_MIN_WINDOW) return null;
     const isOpen = state.modes[side] === "docked" || state.modes[side] === "overlay";
     const wantOpen = request.mode === "open" ? true : request.mode === "close" ? false : !isOpen;
     if (wantOpen) {