Fix MIST gallery scene controls, video positioning and audio overlap

frontend/tests/audio.test.mjs

@@ -93,6 +93,19 @@ test("outgoing tracks also obey the mixer during crossfade", () => {
   assert.equal(elements[0].paused, true);
 });
 
+test("rapid crossfades keep only the current track and its immediate predecessor", () => {
+  const audio = system();
+  audio.play("sound", ["effect"], {});
+  for (const file of ["one", "two", "three", "four"]) {
+    audio.play("music", [file], { fadeout: 2, fadein: 2 });
+    tick(100);
+  }
+  assert.deepEqual(elements.filter(el => !el.paused).map(el => el.src), ["effect", "three", "four"]);
+  audio.stop("music");
+  assert.deepEqual(elements.filter(el => !el.paused).map(el => el.src), ["effect"]);
+  assert.equal(frames.size, 0);
+});
+
 test("stopping during fade-in cancels the old animation", () => {
   const audio = system();
   audio.play("music", ["track"], { fadein: 2 });