fix(studio): a timeline jump no longer starts thumbnail loads it is about to cancel (#5085)

* fix(studio): a timeline jump no longer starts thumbnail loads it is about to cancel

* fix(studio): pump inline after the release batch, and pin the synchronous slot free

packages/studio/src/player/lib/thumbnailScheduler.test.ts

@@ -219,6 +219,26 @@ describe("ThumbnailScheduler", () => {
     expect(scheduler.getDiagnostics()).toMatchObject({ queued: 0, active: 0, leases: 0 });
   });
 
+  it("starts nothing that a release later in the same batch drops", async () => {
+    const scheduler = new ThumbnailScheduler(
+      resolveTimelineViewportBudgets({ concurrentVideoDecodes: 1 }),
+    );
+    const video = (key: string, load: ThumbnailRequest["load"]) =>
+      scheduler.acquire(request(key, load, "visible", { kind: "video" }), vi.fn());
+    const activeLease = video("active", () => deferred<ThumbnailLoadedResult>().promise);
+    const droppedLoad = vi.fn(async () => result("dropped"));
+    const droppedLease = video("dropped", droppedLoad);
+    const keptLoad = vi.fn(async () => result("kept"));
+    video("kept", keptLoad);
+
+    // A timeline jump unmounts its clips in one commit, the loading one first.
+    activeLease.release();
+    droppedLease.release();
+    await flush();
+    expect(droppedLoad).not.toHaveBeenCalled();
+    expect(keptLoad).toHaveBeenCalledTimes(1);
+  });
+
   it("frees an aborted job's slot at once, even when its loader ignores the abort", async () => {
     const scheduler = new ThumbnailScheduler(
       resolveTimelineViewportBudgets({ concurrentVideoDecodes: 1 }),
@@ -231,7 +251,6 @@ describe("ThumbnailScheduler", () => {
     lease.release();
     const nextLoad = vi.fn(async () => result("next"));
     scheduler.acquire(request("next", nextLoad, "visible", { kind: "video" }), vi.fn());
-    await flush();
     expect(nextLoad).toHaveBeenCalledTimes(1);
   });
 

packages/studio/src/player/lib/thumbnailScheduler.ts

@@ -316,7 +316,7 @@ export class ThumbnailScheduler {
       slotFree = true;
       this.activeByBucket[bucket]--;
       this.activeByKind[entry.request.kind]--;
-      this.pump();
+      queueMicrotask(() => this.pump());
     };
     controller.signal.addEventListener("abort", freeSlot, { once: true });
     this.notify(entry);