feat(chat-switch): in-memory message cache helper

apps/desktop/src/lib/messageMemoryCache.test.ts

@@ -0,0 +1,61 @@
+import { afterEach, describe, expect, it } from 'vitest';
+import type { DecryptedMessage } from '@chat-app/shared/chat';
+
+import {
+  __resetForTests,
+  getCachedMessages,
+  hasCachedMessages,
+  setCachedMessages,
+} from './messageMemoryCache';
+
+function msg(id: string): DecryptedMessage {
+  return {
+    id,
+    conversationId: 'conv-1',
+    senderId: 'sender-1',
+    senderDeviceId: null,
+    replyToId: null,
+    editedAt: null,
+    deletedAt: null,
+    createdAt: '2026-05-17T00:00:00Z',
+    plaintext: 'hi ' + id,
+  };
+}
+
+describe('messageMemoryCache', () => {
+  afterEach(() => {
+    __resetForTests();
+  });
+
+  it('returns empty array when nothing is cached', () => {
+    expect(getCachedMessages('unknown')).toEqual([]);
+    expect(hasCachedMessages('unknown')).toBe(false);
+  });
+
+  it('stores and returns messages per conversation', () => {
+    setCachedMessages('a', [msg('m1'), msg('m2')]);
+    expect(getCachedMessages('a').map((m) => m.id)).toEqual(['m1', 'm2']);
+    expect(hasCachedMessages('a')).toBe(true);
+  });
+
+  it('isolates conversations', () => {
+    setCachedMessages('a', [msg('m1')]);
+    setCachedMessages('b', [msg('m9')]);
+    expect(getCachedMessages('a').map((m) => m.id)).toEqual(['m1']);
+    expect(getCachedMessages('b').map((m) => m.id)).toEqual(['m9']);
+  });
+
+  it('overwrites prior cache when set again', () => {
+    setCachedMessages('a', [msg('m1')]);
+    setCachedMessages('a', [msg('m1'), msg('m2')]);
+    expect(getCachedMessages('a').map((m) => m.id)).toEqual(['m1', 'm2']);
+  });
+
+  it('treats an explicit empty list as "cached"', () => {
+    // A conversation that genuinely has zero messages should still be
+    // flagged as cached so the hook skips the loading spinner on re-entry.
+    setCachedMessages('a', []);
+    expect(hasCachedMessages('a')).toBe(true);
+    expect(getCachedMessages('a')).toEqual([]);
+  });
+});

apps/desktop/src/lib/messageMemoryCache.ts

@@ -0,0 +1,37 @@
+// In-memory cache of the most-recently-rendered messages for each
+// conversation. Survives React component unmount/remount (used by
+// `useConversationMessages` to initialize state synchronously when
+// ConversationPage is remounted on chat switch). Session-scoped — lost
+// on full app reload. The SQLite cache (`messageCache.ts`) is still the
+// source of truth for cross-session persistence; this layer just shaves
+// off the round-trip-to-disk spinner flash.
+//
+// Two-tier semantics:
+//   * `hasCachedMessages(id)` returns true even for a known-empty chat
+//     so the hook can suppress the loading spinner on re-entry.
+//   * `getCachedMessages(id)` returns a defensive copy so callers can't
+//     mutate the cached array.
+
+import type { DecryptedMessage } from '@chat-app/shared/chat';
+
+const cache = new Map<string, DecryptedMessage[]>();
+
+export function getCachedMessages(conversationId: string): DecryptedMessage[] {
+  const stored = cache.get(conversationId);
+  return stored ? stored.slice() : [];
+}
+
+export function hasCachedMessages(conversationId: string): boolean {
+  return cache.has(conversationId);
+}
+
+export function setCachedMessages(
+  conversationId: string,
+  messages: DecryptedMessage[],
+): void {
+  cache.set(conversationId, messages.slice());
+}
+
+export function __resetForTests(): void {
+  cache.clear();
+}