fix: go up one folder on Android back in the Files tab

Back inside a Files subfolder switched to the Mail tab. It now closes an
open preview, then leaves selection mode, then goes up one folder, and
only switches tabs at the root. `filesBackStep` holds that order.

src/lib/__tests__/files-back.test.ts

@@ -0,0 +1,23 @@
+import { describe, it, expect } from 'vitest';
+import { filesBackStep } from '../files-back';
+
+describe('filesBackStep', () => {
+  it('leaves the tab only at the root with nothing open', () => {
+    expect(filesBackStep({ previewOpen: false, selecting: false, folderDepth: 0 })).toBe('leave');
+  });
+
+  it('goes up one folder inside a subfolder', () => {
+    expect(filesBackStep({ previewOpen: false, selecting: false, folderDepth: 1 })).toBe('folder-up');
+    expect(filesBackStep({ previewOpen: false, selecting: false, folderDepth: 3 })).toBe('folder-up');
+  });
+
+  it('ends selection mode before changing folders', () => {
+    expect(filesBackStep({ previewOpen: false, selecting: true, folderDepth: 2 })).toBe('clear-selection');
+    expect(filesBackStep({ previewOpen: false, selecting: true, folderDepth: 0 })).toBe('clear-selection');
+  });
+
+  it('closes an open preview before anything else', () => {
+    expect(filesBackStep({ previewOpen: true, selecting: true, folderDepth: 2 })).toBe('close-preview');
+    expect(filesBackStep({ previewOpen: true, selecting: false, folderDepth: 0 })).toBe('close-preview');
+  });
+});

src/lib/files-back.ts

@@ -0,0 +1,19 @@
+/** What Android back does in the Files tab. */
+export type FilesBackStep = 'close-preview' | 'clear-selection' | 'folder-up' | 'leave';
+
+/**
+ * Back works through the tab's own layers, innermost first: an open preview
+ * closes, then selection mode ends (like the header's X), then the folder
+ * goes up one level. Only at the root with nothing open does it `leave`,
+ * i.e. fall through to navigation, which switches to the Mail tab.
+ */
+export function filesBackStep(state: {
+  previewOpen: boolean;
+  selecting: boolean;
+  folderDepth: number;
+}): FilesBackStep {
+  if (state.previewOpen) return 'close-preview';
+  if (state.selecting) return 'clear-selection';
+  if (state.folderDepth > 0) return 'folder-up';
+  return 'leave';
+}

src/screens/FilesScreen.tsx

@@ -53,6 +53,7 @@ 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 { filesBackStep } from '../lib/files-back';
 import type { FileNode } from '../api/types';
 import { spacing, radius, typography, type ThemePalette } from '../theme/tokens';
 import { useColors } from '../theme/colors';
@@ -325,10 +326,6 @@ 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) => {
@@ -459,6 +456,20 @@ export default function FilesScreen() {
     clearSelection();
   }, [clearSelection]);
 
+  // Android back closes the preview, then leaves selection mode, then goes
+  // up one folder; only at the root does it switch to the Mail tab.
+  const backStep = filesBackStep({
+    previewOpen: previewTarget != null,
+    selecting: selectionMode,
+    folderDepth: path.length,
+  });
+  const handleBack = useCallback(() => {
+    if (backStep === 'close-preview') setPreviewTarget(null);
+    else if (backStep === 'clear-selection') clearSelection();
+    else if (backStep === 'folder-up') goBack();
+  }, [backStep, clearSelection, goBack]);
+  useBackWhileFocused(backStep !== 'leave', handleBack);
+
   const toggleView = useCallback(() => {
     const next: FilesViewMode = viewMode === 'list' ? 'grid' : 'list';
     setViewOverride(next);