Stage 7: regression tests for the polished UI (logic + behaviour)

Lock the polish branch's behaviour so a future UI edit surfaces as a failing
assertion to re-agree or fix.

Unit (vitest, node env):
- placement: recallIndex, cellOccupied/isBlankSlot, non-linear direction, the
  single-tile submit default, and placementFromHint blank-fallback / rack-exhausted.
- banner: the marquee scroll-cycle repeat-then-advance, stop(), root-relative and
  multiple links.
- client.GatewayError. Extract the check-word constraints out of Game.svelte into a
  pure lib/checkword.ts (sanitize + canCheck) and cover them.

E2E (playwright mock, Chromium + WebKit):
- commit via the 🏁 control, history slide-down + close, the exchange dialog,
  check-word input sanitising + verdict, resign-to-finished, and the Settings
  board-label mode changing the on-board labels.

ui/src/game/Game.svelte

@@ -15,6 +15,7 @@
   import type { ChatMessage, Direction, EvalResult, MoveRecord, StateView } from '../lib/model';
   import { replay } from '../lib/board';
   import { alphabet, centre, premiumGrid } from '../lib/premiums';
+  import { canCheckWord, sanitizeCheckWord } from '../lib/checkword';
   import {
     BLANK,
     newPlacement,
@@ -311,15 +312,12 @@
     checkOpen = true;
   }
   function onCheckInput(e: Event) {
-    const allowed = new Set(alphabet(variant));
-    const raw = (e.target as HTMLInputElement).value.toUpperCase();
-    checkWord = Array.from(raw).filter((ch) => allowed.has(ch)).slice(0, 15).join('');
+    checkWord = sanitizeCheckWord((e.target as HTMLInputElement).value, alphabet(variant));
   }
   // Check is disabled while cooling down, for an already-checked word, or an out-of-range
   // length. The input filter already restricts to the variant's alphabet.
   function canCheck(): boolean {
-    const w = checkWord.trim();
-    return w.length >= 2 && w.length <= 15 && !checkedWords.has(w.toUpperCase()) && !cooling;
+    return canCheckWord(checkWord, checkedWords.has(checkWord.trim().toUpperCase()), cooling);
   }
   async function runCheck() {
     if (!canCheck()) return;

ui/src/lib/banner.test.ts

@@ -12,6 +12,12 @@ describe('linkify', () => {
     expect(linkify('[x](ftp://evil)')).toBe('x');
     expect(linkify('[y](javascript:boom)')).toBe('y');
   });
+  it('keeps root-relative links and renders several in one string', () => {
+    expect(linkify('go [home](/lobby) or [docs](https://x.com)')).toBe(
+      'go <a href="/lobby" target="_blank" rel="noopener noreferrer">home</a> or ' +
+        '<a href="https://x.com" target="_blank" rel="noopener noreferrer">docs</a>',
+    );
+  });
 });
 
 describe('banner rotator', () => {
@@ -43,4 +49,42 @@ describe('banner rotator', () => {
     expect(scrolled).toBe(1);
     r.stop();
   });
+
+  it('repeats the scroll cycle while under holdMs, then advances', () => {
+    vi.useFakeTimers();
+    const cfg = { holdMs: 2500, edgePauseMs: 100, fadeMs: 10, scrollPxPerSec: 100 };
+    const shown: number[] = [];
+    let scrolls = 0;
+    const r = createBannerRotator(
+      [{ md: 'long' }, { md: 'short' }],
+      { overflowPx: (i) => (i === 0 ? 100 : 0), show: (i) => shown.push(i), scrollTo: () => scrolls++ },
+      cfg,
+    );
+    r.start();
+    vi.advanceTimersByTime(110); // fade(10) + edgePause(100) -> first scroll
+    expect(scrolls).toBe(1);
+    expect(shown).toEqual([0]);
+    vi.advanceTimersByTime(1200); // scrollDur(1000) + edgePause + edgePause -> re-show + second scroll
+    expect(scrolls).toBe(2);
+    expect(shown).toEqual([0, 0]); // re-shown to reset scroll, still item 0 (under holdMs)
+    vi.advanceTimersByTime(10_000);
+    expect(shown).toContain(1); // eventually exceeds holdMs and advances to the fitting message
+    r.stop();
+  });
+
+  it('stop() halts further advancement', () => {
+    vi.useFakeTimers();
+    const cfg = { ...defaultBannerConfig, holdMs: 100, fadeMs: 5, edgePauseMs: 5 };
+    const shown: number[] = [];
+    const r = createBannerRotator(
+      [{ md: 'a' }, { md: 'b' }],
+      { overflowPx: () => 0, show: (i) => shown.push(i), scrollTo: () => {} },
+      cfg,
+    );
+    r.start();
+    vi.advanceTimersByTime(cfg.fadeMs);
+    r.stop();
+    vi.advanceTimersByTime(10_000);
+    expect(shown).toEqual([0]);
+  });
 });

ui/src/lib/checkword.test.ts

@@ -0,0 +1,42 @@
+import { describe, expect, it } from 'vitest';
+import { canCheckWord, MAX_WORD_LEN, sanitizeCheckWord } from './checkword';
+
+const EN = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'.split('');
+
+describe('sanitizeCheckWord', () => {
+  it('upper-cases and keeps only letters of the alphabet', () => {
+    expect(sanitizeCheckWord('ca7t!', EN)).toBe('CAT');
+    expect(sanitizeCheckWord('  Hi 9 ', EN)).toBe('HI');
+  });
+
+  it('drops characters outside the active alphabet', () => {
+    expect(sanitizeCheckWord('cat', ['C', 'A'])).toBe('CA'); // T not in this alphabet
+    const RU = 'КОТ'.split('');
+    expect(sanitizeCheckWord('коt', RU)).toBe('КО'); // cyrillic kept, latin "t" dropped
+  });
+
+  it('caps the length at MAX_WORD_LEN', () => {
+    expect(sanitizeCheckWord('A'.repeat(30), EN)).toHaveLength(MAX_WORD_LEN);
+  });
+});
+
+describe('canCheckWord', () => {
+  it('allows a fresh, in-range word', () => {
+    expect(canCheckWord('CAT', false, false)).toBe(true);
+  });
+
+  it('rejects an out-of-range length', () => {
+    expect(canCheckWord('A', false, false)).toBe(false); // too short
+    expect(canCheckWord('A'.repeat(MAX_WORD_LEN + 1), false, false)).toBe(false); // too long
+  });
+
+  it('rejects an already-checked word or a cooling-down state', () => {
+    expect(canCheckWord('CAT', true, false)).toBe(false);
+    expect(canCheckWord('CAT', false, true)).toBe(false);
+  });
+
+  it('trims surrounding whitespace before measuring length', () => {
+    expect(canCheckWord('  ok  ', false, false)).toBe(true);
+    expect(canCheckWord('  a  ', false, false)).toBe(false);
+  });
+});

ui/src/lib/checkword.ts

@@ -0,0 +1,31 @@
+// Pure helpers for the in-game "check a word" panel: input sanitising and the gate on
+// when a check may be sent. Kept separate from Game.svelte so the constraints (the
+// variant alphabet, the length bounds, the answered-word cache and the cool-down
+// throttle) are unit-testable and stay in lockstep with the UI.
+
+/** The longest word that fits on a standard 15-cell board line. */
+export const MAX_WORD_LEN = 15;
+/** The shortest word worth checking. */
+export const MIN_WORD_LEN = 2;
+
+/**
+ * sanitizeCheckWord upper-cases the raw input and keeps only characters of the active
+ * variant's alphabet, capped at MAX_WORD_LEN — so the field can never hold something the
+ * dictionary could not contain.
+ */
+export function sanitizeCheckWord(raw: string, alphabet: string[]): string {
+  const allowed = new Set(alphabet);
+  return Array.from(raw.toUpperCase())
+    .filter((ch) => allowed.has(ch))
+    .slice(0, MAX_WORD_LEN)
+    .join('');
+}
+
+/**
+ * canCheckWord gates the Check action: the trimmed word must be of valid length, must not
+ * have been answered already (cached), and must not fall inside the cool-down window.
+ */
+export function canCheckWord(word: string, alreadyChecked: boolean, cooling: boolean): boolean {
+  const w = word.trim();
+  return w.length >= MIN_WORD_LEN && w.length <= MAX_WORD_LEN && !alreadyChecked && !cooling;
+}

ui/src/lib/client.test.ts

@@ -0,0 +1,18 @@
+import { describe, expect, it } from 'vitest';
+import { GatewayError } from './client';
+
+describe('GatewayError', () => {
+  it('carries a stable code and is a real Error', () => {
+    const e = new GatewayError('no_hint_available');
+    expect(e).toBeInstanceOf(Error);
+    expect(e.name).toBe('GatewayError');
+    expect(e.code).toBe('no_hint_available');
+    expect(e.message).toBe('no_hint_available'); // message defaults to the code
+  });
+
+  it('keeps a custom message while the code stays the i18n lookup key', () => {
+    const e = new GatewayError('not_your_turn', 'It is not your turn');
+    expect(e.code).toBe('not_your_turn');
+    expect(e.message).toBe('It is not your turn');
+  });
+});

ui/src/lib/placement.test.ts

@@ -1,12 +1,15 @@
 import { describe, expect, it } from 'vitest';
 import {
   BLANK,
+  cellOccupied,
   direction,
+  isBlankSlot,
   newPlacement,
   place,
   placementFromHint,
   rackView,
   recallAt,
+  recallIndex,
   reset,
   toSubmit,
 } from './placement';
@@ -62,6 +65,29 @@ describe('placement state machine', () => {
     expect(toSubmit(place(newPlacement(rack), 0, 7, 7), 'V')?.dir).toBe('V');
     expect(toSubmit(newPlacement(rack))).toBeNull();
   });
+
+  it('recalls a tile by rack index and reports occupied cells / blank slots', () => {
+    let p = place(newPlacement(rack), 0, 7, 7);
+    p = place(p, 1, 7, 8);
+    expect(cellOccupied(p, 7, 7)).toBe(true);
+    expect(cellOccupied(p, 6, 6)).toBe(false);
+    p = recallIndex(p, 0);
+    expect(p.pending.map((t) => t.rackIndex)).toEqual([1]);
+    expect(isBlankSlot(newPlacement(rack), 2)).toBe(true); // '?' slot
+    expect(isBlankSlot(newPlacement(rack), 0)).toBe(false);
+  });
+
+  it('treats a non-linear placement as no inferred direction', () => {
+    let p = place(newPlacement(rack), 0, 7, 7);
+    p = place(p, 1, 8, 8); // diagonal
+    expect(direction(p)).toBeNull();
+  });
+
+  it('defaults a single-tile submit to H without an override', () => {
+    const sub = toSubmit(place(newPlacement(rack), 0, 7, 7));
+    expect(sub?.dir).toBe('H');
+    expect(sub?.tiles).toHaveLength(1);
+  });
 });
 
 describe('placementFromHint', () => {
@@ -78,4 +104,21 @@ describe('placementFromHint', () => {
     expect(p.pending[0]).toMatchObject({ rackIndex: 0, letter: 'C', blank: false });
     expect(p.pending[2]).toMatchObject({ rackIndex: 2, letter: 'B', blank: true });
   });
+
+  it('falls back to a blank slot when the hint letter is not in the rack', () => {
+    const p = placementFromHint([{ row: 7, col: 7, letter: 'Z', blank: false }], ['A', BLANK]);
+    expect(p.pending).toHaveLength(1);
+    expect(p.pending[0]).toMatchObject({ rackIndex: 1, letter: 'Z', blank: true });
+  });
+
+  it('skips hint tiles once the rack is exhausted', () => {
+    const p = placementFromHint(
+      [
+        { row: 7, col: 7, letter: 'A', blank: false },
+        { row: 7, col: 8, letter: 'B', blank: false },
+      ],
+      ['A'],
+    );
+    expect(p.pending.map((t) => t.letter)).toEqual(['A']);
+  });
 });