Fix mobile danmaku canvas scaling

tests/danmaku-canvas-utils.test.ts

@@ -0,0 +1,72 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import {
+  clampDanmakuY,
+  resolveDanmakuCanvasMetrics,
+  scaleDanmakuCoordinate,
+} from '@/lib/player/danmaku-canvas-utils';
+
+test('resolveDanmakuCanvasMetrics prefers layout dimensions over transformed bounding dimensions', () => {
+  const metrics = resolveDanmakuCanvasMetrics({
+    computedWidth: 320,
+    computedHeight: 568,
+    clientWidth: 320,
+    clientHeight: 568,
+    boundingWidth: 568,
+    boundingHeight: 320,
+    devicePixelRatio: 3,
+  });
+
+  assert.deepEqual(metrics, {
+    width: 320,
+    height: 568,
+    dpr: 3,
+    bitmapWidth: 960,
+    bitmapHeight: 1704,
+  });
+});
+
+test('resolveDanmakuCanvasMetrics falls back to bounding dimensions when layout dimensions are unavailable', () => {
+  const metrics = resolveDanmakuCanvasMetrics({
+    boundingWidth: 640,
+    boundingHeight: 360,
+    devicePixelRatio: 2,
+  });
+
+  assert.deepEqual(metrics, {
+    width: 640,
+    height: 360,
+    dpr: 2,
+    bitmapWidth: 1280,
+    bitmapHeight: 720,
+  });
+});
+
+test('resolveDanmakuCanvasMetrics ignores invalid sizes and invalid device pixel ratios', () => {
+  const metrics = resolveDanmakuCanvasMetrics({
+    computedWidth: 0,
+    computedHeight: Number.NaN,
+    clientWidth: 375,
+    clientHeight: 211,
+    devicePixelRatio: Number.NaN,
+  });
+
+  assert.deepEqual(metrics, {
+    width: 375,
+    height: 211,
+    dpr: 1,
+    bitmapWidth: 375,
+    bitmapHeight: 211,
+  });
+});
+
+test('scaleDanmakuCoordinate preserves relative placement after resize', () => {
+  assert.equal(scaleDanmakuCoordinate(160, 320, 640), 320);
+  assert.equal(scaleDanmakuCoordinate(40, 320, 240), 30);
+});
+
+test('clampDanmakuY keeps comments inside the effective display area', () => {
+  assert.equal(clampDanmakuY(4, 20, 120), 20);
+  assert.equal(clampDanmakuY(200, 20, 120), 112);
+  assert.equal(clampDanmakuY(64, 20, 120), 64);
+});