*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

body {
  font-family: 'Clear Sans', 'Helvetica Neue', Arial, sans-serif;
  background: #faf8ef;
  display: flex;
  justify-content: center;
  align-items: flex-start;
  min-height: 100vh;
  padding: 16px;
  touch-action: none;
  overscroll-behavior: none;
  overflow: hidden;
  user-select: none;
  -webkit-user-select: none;
}

#app {
  width: 100%;
  max-width: 420px;
  position: relative;
}

#header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 12px;
}

h1 {
  font-size: 3rem;
  font-weight: 700;
  color: #776e65;
}

.score-box {
  background: #bbada0;
  border-radius: 6px;
  padding: 8px 16px;
  text-align: center;
  min-width: 80px;
}

.score-label {
  display: block;
  font-size: 0.65rem;
  font-weight: 700;
  color: #eee4da;
  letter-spacing: 0.05em;
}

#score {
  display: block;
  font-size: 1.4rem;
  font-weight: 700;
  color: #fff;
}

#controls {
  display: flex;
  justify-content: flex-end;
  margin-bottom: 12px;
}

#new-game-btn {
  background: #8f7a66;
  color: #f9f6f2;
  border: none;
  border-radius: 6px;
  padding: 10px 20px;
  font-family: inherit;
  font-size: 0.9rem;
  font-weight: 700;
  cursor: pointer;
  transition: background 0.15s;
}

#new-game-btn:hover { background: #7a6856; }

/* ─── Board ──────────────────────────────────────────────────────────────── */
#board-container {
  position: relative;
  background: #bbada0;
  border-radius: 10px;
  padding: 10px;
}

/* Static background cells */
#board {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 10px;
}

.cell-bg {
  aspect-ratio: 1;
  border-radius: 6px;
  background: rgba(238, 228, 218, 0.35);
}

/* ─── Tile layer ─────────────────────────────────────────────────────────── */
#tile-layer {
  position: absolute;
  inset: 0;
  pointer-events: none;
}

/* Tiles: absolutely positioned, slide via left/top transitions */
.tile {
  position: absolute;
  width: calc((100% - 50px) / 4);
  height: calc((100% - 50px) / 4);
  left: calc(10px + var(--c) * ((100% - 50px) / 4 + 10px));
  top:  calc(10px + var(--r) * ((100% - 50px) / 4 + 10px));
  border-radius: 6px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-weight: 700;
  font-size: 1.8rem;
  transition: left 0.12s ease, top 0.12s ease;
  will-change: left, top;
  z-index: 1;
}

/* New tile: spring pop-in from scale 0 */
.tile-new {
  animation: tile-appear 0.2s cubic-bezier(0.34, 1.56, 0.64, 1) backwards;
}

/* Merged tile: spring bounce, on top of sliding loser */
.tile-merged {
  animation: tile-pop 0.25s cubic-bezier(0.34, 1.56, 0.64, 1);
  z-index: 2;
}

@keyframes tile-appear {
  0%   { transform: scale(0); opacity: 0; }
  100% { transform: scale(1); opacity: 1; }
}

@keyframes tile-pop {
  0%   { transform: scale(0.85); }
  100% { transform: scale(1); }
}

/* ─── Overlay ──────────────────────────────────────────────────────────────── */
#overlay {
  position: absolute;
  inset: 0;
  background: rgba(238, 228, 218, 0.75);
  border-radius: 10px;
  display: flex;
  align-items: center;
  justify-content: center;
  z-index: 10;
}

#overlay.hidden { display: none; }

#overlay-content {
  text-align: center;
  padding: 24px;
}

#overlay-title {
  font-size: 2rem;
  font-weight: 700;
  color: #776e65;
  margin-bottom: 16px;
}

#overlay-btn {
  background: #8f7a66;
  color: #f9f6f2;
  border: none;
  border-radius: 6px;
  padding: 12px 28px;
  font-family: inherit;
  font-size: 1rem;
  font-weight: 700;
  cursor: pointer;
  margin-top: 8px;
  transition: background 0.15s;
}

#overlay-btn:hover { background: #7a6856; }

/* ─── Responsive ──────────────────────────────────────────────────────────── */
@media (max-width: 480px) {
  h1 { font-size: 2rem; }
  .tile { font-size: 1.4rem; }
}

/* Landscape / short viewport: compact header so board fits */
@media (max-height: 500px) {
  body { padding: 8px; align-items: center; }
  #header { margin-bottom: 6px; }
  h1 { font-size: 1.4rem; }
  #controls { margin-bottom: 6px; }
  #new-game-btn { padding: 6px 14px; font-size: 0.8rem; }
  .score-box { padding: 4px 10px; }
  #score { font-size: 1rem; }
  .tile { font-size: 1.1rem; }
}