docs/ARCHITECTURE.md

@@ -585,7 +585,10 @@ and **invitation-update** carry the full invitation, and the client upserts a st
 drops a terminal one (started, declined, cancelled, expired) — the invitations list is a delta channel
 too, fresh from any screen without a refetch. The move-commit **response** (`submit_play` / `pass` /
 `exchange` / `resign`) likewise returns the actor's own refilled rack and bag size, so the mover
-renders the next turn without a self-refetch. The `notify` package owns the FlatBuffers encoding
+renders the next turn without a self-refetch. Beyond that event-driven warming, the lobby
+**preloads** the player's ongoing games — each one's `game.state`, `game.history` and saved
+**draft** — into that same per-game cache, so opening one from the lobby is instant and a saved
+composition paints already on the board (no rack→board step). The `notify` package owns the FlatBuffers encoding
 (fed wire-agnostic input structs by the domain services) and the gateway forwards every payload
 verbatim. Auto-match needs no match poll — `Enqueue` returns the game the player enters
 synchronously, and an opponent later taking the open seat arrives as the in-app **opponent-joined**

ui/src/game/Game.svelte

@@ -19,7 +19,7 @@
   import { variantNameKey } from '../lib/variants';
   import { alphabetLetters, hasAlphabet } from '../lib/alphabet';
   import { shareOrDownloadGcg } from '../lib/share';
-  import { getCachedGame, setCachedGame, type CachedGame } from '../lib/gamecache';
+  import { getCachedGame, setCachedGame, setCachedDraft, type CachedGame } from '../lib/gamecache';
   import { patchLobbyGame } from '../lib/lobbycache';
   import { applyGameOver, applyMoveDelta, applyOpponentJoined, type DeltaResult } from '../lib/gamedelta';
   import { telegramClosingConfirmation, telegramHaptic } from '../lib/telegram';
@@ -136,18 +136,21 @@
       // Ask for the alphabet table only on a per-variant cache miss (the first open of a
       // game whose variant the client has not cached yet); steady-state polls omit it.
       const includeAlphabet = !view || !hasAlphabet(view.game.variant);
-      const [st, hist] = await Promise.all([
+      // Fetch the saved draft alongside state and history (best-effort) so the composition is
+      // applied in the same tick the board appears — never as a second, visible rack→board step.
+      const [st, hist, draft] = await Promise.all([
         gateway.gameState(id, includeAlphabet),
         gateway.gameHistory(id),
+        gateway.draftGet(id).catch(() => ''),
       ]);
       view = st;
       moves = hist.moves;
-      setCachedGame(id, st, hist.moves);
+      setCachedGame(id, st, hist.moves, draft);
       // Mirror the fresh status into the lobby snapshot so returning there shows it without a
       // stale flash before the lobby's own background refresh lands.
       patchLobbyGame(st.game);
       selected = null;
-      await applyDraft(st);
+      applyDraft(st, draft);
       recompute();
       refreshRecent();
     } catch (e) {
@@ -156,29 +159,29 @@
   }
   let draftSaveTimer: ReturnType<typeof setTimeout> | null = null;
   // scheduleDraftSave persists the composition (rack order + pending tiles) after a short
-  // debounce; best-effort, so a failed save never interrupts play.
+  // debounce; best-effort, so a failed save never interrupts play. The cache is updated at once
+  // so leaving and re-entering the game in the same session paints the latest composition.
   function scheduleDraftSave() {
+    const json = serializeDraft(rackIds, placement.pending);
+    setCachedDraft(id, json);
     if (draftSaveTimer) clearTimeout(draftSaveTimer);
     draftSaveTimer = setTimeout(() => {
-      void gateway.draftSave(id, serializeDraft(rackIds, placement.pending)).catch(() => {});
+      void gateway.draftSave(id, json).catch(() => {});
     }, 500);
   }
   // applyDraft restores the player's saved composition over a freshly loaded state: the rack
   // order (when still a valid permutation of the rack) and the board tiles whose cell is still
-  // free. Best-effort — a draft fetch never blocks opening the game.
-  async function applyDraft(st: StateView) {
+  // free. It takes the already-fetched draft JSON (load and the warm onMount supply it), so the
+  // composition is applied synchronously — no extra fetch, no second rack→board step.
+  function applyDraft(st: StateView, draftJson: string) {
     let order = st.rack.map((_, i) => i);
     let tiles: Tile[] = [];
-    try {
-      const parsed = parseDraft(await gateway.draftGet(id));
+    const parsed = parseDraft(draftJson);
     if (parsed) {
       order = validRackOrder(parsed.rackOrder, st.rack.length) ?? order;
       const committed = replay(moves);
       tiles = liveDraftTiles(parsed.tiles, (r, c) => !!committed[r]?.[c]);
     }
-    } catch {
-      /* best-effort */
-    }
     rackIds = order;
     const rack = order.map((i) => st.rack[i]);
     placement = tiles.length ? placementFromHint(tiles, rack) : newPlacement(rack);
@@ -192,8 +195,9 @@
     if (cached) {
       view = cached.view;
       moves = cached.moves;
-      placement = newPlacement(cached.view.rack);
-      rackIds = cached.view.rack.map((_, i) => i);
+      // Apply the cached draft synchronously so a warm/preloaded open paints the pending tiles
+      // already on the board (no full-rack-then-jump). load() then refreshes in the background.
+      applyDraft(cached.view, cached.draft ?? '');
       refreshRecent();
     }
     void load();
@@ -576,7 +580,9 @@
   function applyMoveResult(r: MoveResult) {
     view = { game: r.game, seat: r.move.player, rack: r.rack, bagLen: r.bagLen, hintsRemaining: view?.hintsRemaining ?? 0 };
     moves = [...moves, r.move];
-    setCachedGame(id, view, moves);
+    // A committed move clears the actor's draft on the server, so clear the cached draft too;
+    // otherwise a same-session re-entry would briefly re-apply the now-stale composition.
+    setCachedGame(id, view, moves, '');
     patchLobbyGame(r.game);
     rackIds = r.rack.map((_, i) => i);
     placement = newPlacement(r.rack);

ui/src/lib/gamecache.test.ts

@@ -0,0 +1,87 @@
+import { beforeEach, describe, expect, it } from 'vitest';
+import { clearGameCache, getCachedGame, setCachedDraft, setCachedGame } from './gamecache';
+import type { GameView, MoveRecord, StateView } from './model';
+
+function gameView(id: string): GameView {
+  return {
+    id,
+    variant: 'scrabble_en',
+    dictVersion: 'v1',
+    status: 'active',
+    players: 2,
+    toMove: 0,
+    turnTimeoutSecs: 300,
+    multipleWordsPerTurn: true,
+    moveCount: 0,
+    endReason: '',
+    lastActivityUnix: 0,
+    seats: [],
+  };
+}
+
+function view(id: string, rack: string[] = ['A', 'B']): StateView {
+  return { game: gameView(id), seat: 0, rack, bagLen: 50, hintsRemaining: 1 };
+}
+
+function move(player: number): MoveRecord {
+  return { player, action: 'play', dir: 'H', mainRow: 7, mainCol: 7, tiles: [], words: ['AB'], count: 0, score: 10, total: 10 };
+}
+
+beforeEach(() => clearGameCache());
+
+describe('setCachedGame', () => {
+  it('stores state, history and draft', () => {
+    setCachedGame('g1', view('g1'), [move(0)], '{"x":1}');
+    const c = getCachedGame('g1');
+    expect(c?.view.game.id).toBe('g1');
+    expect(c?.moves).toHaveLength(1);
+    expect(c?.draft).toBe('{"x":1}');
+  });
+
+  it('leaves draft undefined when none is given for a new entry', () => {
+    setCachedGame('g1', view('g1'), []);
+    expect(getCachedGame('g1')?.draft).toBeUndefined();
+  });
+
+  it('preserves the cached draft when draft is omitted (the live-event delta path)', () => {
+    setCachedGame('g1', view('g1'), [], 'DRAFT');
+    // A delta advances view+moves without refetching the draft; it must not be dropped.
+    setCachedGame('g1', view('g1', ['C', 'D']), [move(1)]);
+    const c = getCachedGame('g1');
+    expect(c?.draft).toBe('DRAFT'); // preserved
+    expect(c?.view.rack).toEqual(['C', 'D']); // view advanced
+    expect(c?.moves).toHaveLength(1);
+  });
+
+  it('clears the draft when passed an empty string (a committed move)', () => {
+    setCachedGame('g1', view('g1'), [], 'DRAFT');
+    setCachedGame('g1', view('g1'), [move(0)], '');
+    expect(getCachedGame('g1')?.draft).toBe('');
+  });
+});
+
+describe('setCachedDraft', () => {
+  it('updates only the draft of an already-cached game', () => {
+    setCachedGame('g1', view('g1', ['A']), [move(0)], 'OLD');
+    setCachedDraft('g1', 'NEW');
+    const c = getCachedGame('g1');
+    expect(c?.draft).toBe('NEW');
+    expect(c?.view.rack).toEqual(['A']); // unchanged
+    expect(c?.moves).toHaveLength(1);
+  });
+
+  it('is a no-op when the game is not cached', () => {
+    setCachedDraft('absent', 'X');
+    expect(getCachedGame('absent')).toBeUndefined();
+  });
+});
+
+describe('clearGameCache', () => {
+  it('drops every cached game', () => {
+    setCachedGame('g1', view('g1'), []);
+    setCachedGame('g2', view('g2'), []);
+    clearGameCache();
+    expect(getCachedGame('g1')).toBeUndefined();
+    expect(getCachedGame('g2')).toBeUndefined();
+  });
+});

ui/src/lib/gamecache.ts

@@ -1,15 +1,21 @@
-// In-memory per-game cache. A game the player has opened once is kept here so a
-// later re-entry renders instantly from the cache while a fresh fetch updates it in
-// the background — removing the blank "loading" flash and the full redraw on every
-// lobby <-> game navigation. It is intentionally process-memory only (no persistence):
-// stale entries are corrected by the background refresh, and the cache is cleared on
-// logout.
+// In-memory per-game cache. A game the player has opened once — or one warmed ahead of time
+// by the lobby preload (see lib/preload) — is kept here so opening it renders instantly from
+// the cache while a fresh fetch updates it in the background, removing the blank "loading"
+// flash and the full redraw on every lobby <-> game navigation. It is intentionally
+// process-memory only (no persistence): stale entries are corrected by the background
+// refresh, and the cache is cleared on logout.
 
 import type { MoveRecord, StateView } from './model';
 
 export interface CachedGame {
   view: StateView;
   moves: MoveRecord[];
+  /**
+   * The player's saved draft (composition) as the opaque JSON the server stores, so a warm
+   * open paints the pending tiles already on the board with no rack→board jump. An empty
+   * string means no draft; undefined means it has not been fetched for this entry yet.
+   */
+  draft?: string;
 }
 
 const cache = new Map<string, CachedGame>();
@@ -19,9 +25,22 @@ export function getCachedGame(id: string): CachedGame | undefined {
   return cache.get(id);
 }
 
-/** setCachedGame stores the latest state+history for a game. */
-export function setCachedGame(id: string, view: StateView, moves: MoveRecord[]): void {
-  cache.set(id, { view, moves });
+/**
+ * setCachedGame stores the latest state+history for a game. Omitting draft preserves the
+ * cached composition: the live-event delta path advances view+moves without refetching the
+ * draft and must not drop it, while passing an empty string clears it (a committed move does,
+ * mirroring the server).
+ */
+export function setCachedGame(id: string, view: StateView, moves: MoveRecord[], draft?: string): void {
+  const prev = cache.get(id);
+  cache.set(id, { view, moves, draft: draft ?? prev?.draft });
+}
+
+/** setCachedDraft updates only the saved draft of an already-cached game (a no-op otherwise),
+ *  keeping the cache coherent with the draft persisted while composing. */
+export function setCachedDraft(id: string, draft: string): void {
+  const prev = cache.get(id);
+  if (prev) cache.set(id, { ...prev, draft });
 }
 
 /** clearGameCache drops every cached game (called on logout). */

ui/src/lib/preload.test.ts

@@ -0,0 +1,71 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import type { GameView, StateView } from './model';
+
+// Mock the gateway singleton so preload's fan-out is observed without a transport.
+const mocks = vi.hoisted(() => ({
+  gameState: vi.fn(),
+  gameHistory: vi.fn(),
+  draftGet: vi.fn(),
+}));
+vi.mock('./gateway', () => ({
+  gateway: { gameState: mocks.gameState, gameHistory: mocks.gameHistory, draftGet: mocks.draftGet },
+}));
+
+import { preloadGames } from './preload';
+import { clearGameCache, getCachedGame, setCachedGame } from './gamecache';
+
+function gameView(id: string, status: GameView['status'] = 'active'): GameView {
+  return {
+    id,
+    variant: 'scrabble_en',
+    dictVersion: 'v1',
+    status,
+    players: 2,
+    toMove: 0,
+    turnTimeoutSecs: 300,
+    multipleWordsPerTurn: true,
+    moveCount: 0,
+    endReason: '',
+    lastActivityUnix: 0,
+    seats: [],
+  };
+}
+
+function stateView(id: string): StateView {
+  return { game: gameView(id), seat: 0, rack: ['A', 'B'], bagLen: 50, hintsRemaining: 1 };
+}
+
+beforeEach(() => {
+  clearGameCache();
+  mocks.gameState.mockReset().mockImplementation((id: string) => Promise.resolve(stateView(id)));
+  mocks.gameHistory.mockReset().mockImplementation((id: string) => Promise.resolve({ gameId: id, moves: [] }));
+  mocks.draftGet.mockReset().mockImplementation((id: string) => Promise.resolve(id === 'g1' ? 'DRAFT1' : ''));
+});
+
+describe('preloadGames', () => {
+  it('warms ongoing, uncached games with state, history and draft', async () => {
+    await preloadGames([gameView('g1'), gameView('g2')]);
+    expect(getCachedGame('g1')?.view.game.id).toBe('g1');
+    expect(getCachedGame('g1')?.draft).toBe('DRAFT1');
+    expect(getCachedGame('g2')?.draft).toBe('');
+  });
+
+  it('skips finished games', async () => {
+    await preloadGames([gameView('done', 'finished')]);
+    expect(getCachedGame('done')).toBeUndefined();
+    expect(mocks.gameState).not.toHaveBeenCalled();
+  });
+
+  it('skips games already in the cache (kept fresh by the live stream)', async () => {
+    setCachedGame('g1', stateView('g1'), [], 'KEEP');
+    await preloadGames([gameView('g1'), gameView('g2')]);
+    expect(mocks.gameState).toHaveBeenCalledTimes(1);
+    expect(mocks.gameState).toHaveBeenCalledWith('g2', expect.any(Boolean));
+    expect(getCachedGame('g1')?.draft).toBe('KEEP'); // untouched
+  });
+
+  it('does nothing for an empty list', async () => {
+    await preloadGames([]);
+    expect(mocks.gameState).not.toHaveBeenCalled();
+  });
+});

ui/src/lib/preload.ts

@@ -0,0 +1,50 @@
+// Lobby preload: warm the per-game cache for the player's ongoing games ahead of time, so
+// opening one from the lobby renders instantly — no "loading" flash and no rack→board jump
+// (the saved draft is fetched and cached alongside state and history). It is best-effort and
+// bounded; finished games are skipped (rarely reopened, no draft), as are games already cached
+// (the live stream keeps those fresh) or currently being warmed.
+
+import { gateway } from './gateway';
+import { getCachedGame, setCachedGame } from './gamecache';
+import { hasAlphabet } from './alphabet';
+import type { GameView } from './model';
+
+// Cap on games warmed at once: enough to cover a typical lobby quickly without a request burst.
+const MAX_CONCURRENT = 4;
+// Games being warmed right now, so overlapping calls (the lobby refreshes on every live event)
+// never fetch the same game twice.
+const inflight = new Set<string>();
+
+/**
+ * preloadGames warms the cache for the given ongoing games: it fetches each game's state,
+ * history and saved draft in parallel and stores them, skipping finished games and any already
+ * cached or in flight. Concurrency is bounded and failures are swallowed (the game just
+ * cold-loads on open). It resolves once every queued game has settled.
+ */
+export async function preloadGames(games: GameView[]): Promise<void> {
+  const queue = games.filter(
+    (g) => g.status !== 'finished' && !getCachedGame(g.id) && !inflight.has(g.id),
+  );
+  if (queue.length === 0) return;
+  for (const g of queue) inflight.add(g.id);
+  let next = 0;
+  const worker = async (): Promise<void> => {
+    while (next < queue.length) {
+      const g = queue[next++];
+      try {
+        const [st, hist, draft] = await Promise.all([
+          gateway.gameState(g.id, !hasAlphabet(g.variant)),
+          gateway.gameHistory(g.id),
+          gateway.draftGet(g.id).catch(() => ''),
+        ]);
+        // A live event may have seeded or advanced this entry while we fetched; don't clobber it.
+        if (!getCachedGame(g.id)) setCachedGame(g.id, st, hist.moves, draft);
+      } catch {
+        /* best-effort: leave the game to cold-load on open */
+      } finally {
+        inflight.delete(g.id);
+      }
+    }
+  };
+  await Promise.all(Array.from({ length: Math.min(MAX_CONCURRENT, queue.length) }, worker));
+}

ui/src/screens/Lobby.svelte

@@ -9,6 +9,7 @@
   import { t, type MessageKey } from '../lib/i18n/index.svelte';
   import { resultBadge } from '../lib/result';
   import { getLobby, setLobby } from '../lib/lobbycache';
+  import { preloadGames } from '../lib/preload';
   import { groupGames } from '../lib/lobbysort';
   import type { AccountRef, GameView, Invitation } from '../lib/model';
 
@@ -28,6 +29,10 @@
         app.notifications = incoming.length;
       }
       setLobby({ games, invitations, incoming });
+      // Warm the cache for the ongoing games so opening one from the lobby is instant. The list
+      // just loaded, so the connection is up; the call is non-blocking and re-running it on each
+      // lobby refresh cheaply warms any newly appeared game (already-cached ones are skipped).
+      if (connection.online) void preloadGames(games);
     } catch (e) {
       handleError(e);
     }