fix: clear the Files selection on Android back instead of switching to Mail

src/lib/__tests__/use-back-while-focused.test.ts

@@ -0,0 +1,58 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+const handlers: Array<() => boolean> = [];
+const remove = vi.fn();
+
+vi.mock('react-native', () => ({
+  BackHandler: {
+    addEventListener: vi.fn((_event: string, handler: () => boolean) => {
+      handlers.push(handler);
+      return { remove };
+    }),
+  },
+}));
+
+// Run the focus effect as if the screen had just gained focus and keep its
+// cleanup, which react-navigation calls on blur.
+let blur: void | (() => void);
+vi.mock('@react-navigation/native', () => ({
+  useFocusEffect: (effect: () => void | (() => void)) => {
+    blur = effect();
+  },
+}));
+
+vi.mock('react', () => ({ useCallback: <T,>(fn: T) => fn }));
+
+import { BackHandler } from 'react-native';
+import { useBackWhileFocused } from '../use-back-while-focused';
+
+beforeEach(() => {
+  vi.clearAllMocks();
+  handlers.length = 0;
+  blur = undefined;
+});
+
+describe('useBackWhileFocused', () => {
+  it('runs onBack and consumes the back press while active', () => {
+    const onBack = vi.fn();
+    useBackWhileFocused(true, onBack);
+
+    expect(BackHandler.addEventListener).toHaveBeenCalledWith('hardwareBackPress', expect.any(Function));
+    expect(handlers[0]()).toBe(true);
+    expect(onBack).toHaveBeenCalledTimes(1);
+  });
+
+  it('leaves the back press to navigation while inactive', () => {
+    useBackWhileFocused(false, vi.fn());
+
+    expect(BackHandler.addEventListener).not.toHaveBeenCalled();
+  });
+
+  it('stops listening when the screen loses focus', () => {
+    useBackWhileFocused(true, vi.fn());
+
+    expect(typeof blur).toBe('function');
+    (blur as () => void)();
+    expect(remove).toHaveBeenCalledTimes(1);
+  });
+});

src/lib/use-back-while-focused.ts

@@ -0,0 +1,23 @@
+import { useCallback } from 'react';
+import { BackHandler } from 'react-native';
+import { useFocusEffect } from '@react-navigation/native';
+
+/**
+ * While `active` and the screen is focused, Android's hardware back runs
+ * `onBack` instead of navigating (e.g. to leave a selection mode first).
+ * Tab screens stay mounted while another tab is shown, so the listener is
+ * tied to focus: a plain BackHandler listener would also swallow the back
+ * presses meant for the other tabs.
+ */
+export function useBackWhileFocused(active: boolean, onBack: () => void): void {
+  useFocusEffect(
+    useCallback(() => {
+      if (!active) return undefined;
+      const subscription = BackHandler.addEventListener('hardwareBackPress', () => {
+        onBack();
+        return true;
+      });
+      return () => subscription.remove();
+    }, [active, onBack]),
+  );
+}

src/screens/FilesScreen.tsx

@@ -52,6 +52,7 @@ import { jmapClient } from '../api/jmap-client';
 import { downloadAttachment, shareAttachment } from '../lib/email-export';
 import { secureFetch } from '../lib/client-cert';
 import { getUniqueName } from '../lib/filenode-name';
+import { useBackWhileFocused } from '../lib/use-back-while-focused';
 import type { FileNode } from '../api/types';
 import { spacing, radius, typography, type ThemePalette } from '../theme/tokens';
 import { useColors } from '../theme/colors';
@@ -321,6 +322,10 @@ export default function FilesScreen() {
 
   const clearSelection = useCallback(() => setSelection(new Set()), []);
 
+  // Android back leaves selection mode first, like the header's X, instead
+  // of switching to the Mail tab.
+  useBackWhileFocused(selectionMode, clearSelection);
+
   // Shared-with-me rows stay out of multi-select: batch delete routes to our
   // own account and would fail (or worse, mismatch) on namespaced ids.
   const toggleSelect = useCallback((row: FileRow) => {