fix: only show the unified mailbox when it can hold more than one account's mail #843

The drawer showed "All inboxes" and the Unified section as soon as a
second account was signed in, but with cross-account off the unified
views only cover the active account, so they repeated its own folders.
They now follow the webmail's rule: shown with cross-account on and
several accounts, or when group inboxes are merged in.

src/components/SidebarDrawer.tsx

@@ -26,6 +26,7 @@ import {
   buildMailboxTree, flattenVisible, mailboxSubtreeIds, ownMailboxes, type MailboxNode,
 } from '../lib/mailbox-tree';
 import { localizeMailboxName } from '../lib/mailbox-label';
+import { showUnifiedSection } from '../lib/unified-section';
 import { generateAvatarColor, getAccountInitials } from '../lib/avatar-utils';
 import { jmapClient } from '../api/jmap-client';
 import {
@@ -631,12 +632,15 @@ export default function SidebarDrawer({ visible, onClose }: SidebarDrawerProps)
   };
 
   // ── Unified rows ──────────────────────────────────────────────────────
-  // Shown with one account as soon as it has group inboxes to merge, and
+  // Shown only when the views hold more than this account's own folders:
-  // always with several accounts. Counts are projected from the mailbox
+  // cross-account with several accounts, or group inboxes to merge (#843).
-  // lists we hold: the active account's (own + shared) plus, cross-account,
+  // Counts are projected from the mailbox lists we hold: the active
-  // the tucked-away snapshots of the other accounts.
+  // account's (own + shared) plus, cross-account, the tucked-away snapshots
+  // of the other accounts.
   const hasSharedInbox = mailboxes.some((m) => m.isShared && m.role === 'inbox');
-  const showUnified = accounts.length > 1 || (includeGroupInUnified && hasSharedInbox);
+  const showUnified = showUnifiedSection({
+    accountCount: accounts.length, unifiedCrossAccount, includeGroupInUnified, hasSharedInbox,
+  });
   const unifiedCounts = React.useMemo(() => {
     const pools: Mailbox[][] = [mailboxes];
     if (unifiedCrossAccount) {

src/lib/__tests__/unified-section.test.ts

@@ -0,0 +1,21 @@
+import { describe, expect, it } from 'vitest';
+import { showUnifiedSection } from '../unified-section';
+
+const base = { accountCount: 1, unifiedCrossAccount: false, includeGroupInUnified: true, hasSharedInbox: false };
+
+describe('showUnifiedSection (#843)', () => {
+  it('hides it for a second account while cross-account is off', () => {
+    expect(showUnifiedSection({ ...base, accountCount: 2 })).toBe(false);
+  });
+
+  it('shows it once cross-account is on with several accounts', () => {
+    expect(showUnifiedSection({ ...base, accountCount: 2, unifiedCrossAccount: true })).toBe(true);
+    // Cross-account with a single account has nothing to merge.
+    expect(showUnifiedSection({ ...base, unifiedCrossAccount: true })).toBe(false);
+  });
+
+  it('shows it for group inboxes only when they are merged in', () => {
+    expect(showUnifiedSection({ ...base, hasSharedInbox: true })).toBe(true);
+    expect(showUnifiedSection({ ...base, hasSharedInbox: true, includeGroupInUnified: false })).toBe(false);
+  });
+});

src/lib/unified-section.ts

@@ -0,0 +1,20 @@
+/**
+ * Whether the drawer offers the unified mailbox (the "All inboxes" row and
+ * the Unified section), following the webmail's rule (#843): only when the
+ * unified views will hold more than the open account's own folders.
+ *
+ * - Cross-account on and a second account signed in: every account's mail.
+ * - Group inboxes merged in and the account has one: own plus group mail.
+ *
+ * A second account alone is not enough: with cross-account off the views
+ * cover only the active account, so "All inboxes" merely repeated Inbox.
+ */
+export function showUnifiedSection(opts: {
+  accountCount: number;
+  unifiedCrossAccount: boolean;
+  includeGroupInUnified: boolean;
+  hasSharedInbox: boolean;
+}): boolean {
+  const crossAccountActive = opts.unifiedCrossAccount && opts.accountCount > 1;
+  return crossAccountActive || (opts.includeGroupInUnified && opts.hasSharedInbox);
+}