fix: a screen's video no longer flickers and its button stays clickable while a timer redraws the screen

MIST's cutscene screen has a 0.1s timer that changes a variable, and every change rebuilt the whole
screen: the <video> restarted each time and "Skip" was replaced between press and release, so the
click never landed. Videos (Movie(play=...) and the movie channel picture) are now kept per shown
screen and resumed if the browser paused them, and a redraw requested while a pointer is down waits
for its release.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

frontend/tests/screens.test.mjs

@@ -7,7 +7,7 @@ import { parseHTML } from "linkedom";
 
 const { window, document } = parseHTML("<!doctype html><html><body></body></html>");
 Object.assign(globalThis, { window, document, HTMLElement: window.HTMLElement, HTMLInputElement: window.HTMLInputElement,
-  HTMLImageElement: window.HTMLImageElement, Image: window.Image });
+  HTMLImageElement: window.HTMLImageElement, HTMLVideoElement: window.HTMLVideoElement ?? class {}, Image: window.Image });
 window.setTimeout = setTimeout;
 window.clearTimeout = clearTimeout;
 globalThis.requestAnimationFrame = (cb) => setTimeout(() => cb(performance.now()), 1);
@@ -271,3 +271,26 @@ test("an `if` that only sets properties picks the image; decompiled comma action
   assert.equal(store.mood_clicked, 1);
   assert.equal(store.mood_done, true);
 });
+
+test("a redraw keeps the playing video, and one held down button is not rebuilt under the pointer", async () => {
+  const movie = { w: "add", args: [{ k: "call", f: { k: "name", id: "Movie" }, a: [], kw: { play: { k: "const", v: "intro.webm" } } }],
+                  props: {}, body: [] };
+  const { manager } = setup();
+  manager.screens.cutscene = { params: [], props: {}, defaults: [], body: [movie] };
+  manager.show("cutscene", [], {});
+  const first = manager.layer.querySelector("video");
+  assert.ok(first, "the video is on the screen");
+  for (let i = 0; i < 3; i++) { manager.refresh(); await wait(30); }
+  assert.equal(manager.layer.querySelector("video"), first, "the same <video> after redraws");
+  manager.hide("cutscene");
+  manager.show("cutscene", [], {});
+  assert.notEqual(manager.layer.querySelector("video"), first, "a screen shown again starts a new video");
+  // a redraw requested while a pointer is down waits for its release
+  window.dispatchEvent(new window.Event("pointerdown"));
+  const during = manager.layer.querySelector("video");
+  manager.refresh();
+  await wait(30);
+  assert.equal(manager.layer.querySelector("video"), during);
+  window.dispatchEvent(new window.Event("pointerup"));
+  await wait(30);
+});