Preview: the Extended Viewer startup test doesn't race the first frame (#947)

startup_finishes_when_extended_viewer_frames_are_larger failed on busy
machines. The harness clock moves 1/30 s per step however long a frame
takes, so a slow first frame could use up the startup's limits in
harness time. The test now waits for the first frame in real time
before stepping on, as it is about the wait for the frames ahead, and
checks the fake device was started and not stopped.

crates/ui-egui/tests/ui_audio_scrub.rs

@@ -372,9 +372,28 @@ fn startup_finishes_when_extended_viewer_frames_are_larger() {
     assert!((budget as f64 * 0.9 / real_bytes as f64) < ahead as f64, "real frames ({size:?}) fit fewer than {ahead}");
     h.state().frames.set_budget(budget);
     h.state().frames.clear();
-    assert!(until_started(&mut h), "the preview left its startup");
+    let now = h.ctx.input(|i| i.time);
+    h.state_mut().play(now);
+    h.step();
+    // The first frame renders before the harness clock moves on: each step is 1/30 s however long
+    // it takes, so on a busy machine the first frame could take longer than STARTUP_LIMIT of that
+    // clock, and this test is about the wait for the frames ahead.
+    let deadline = std::time::Instant::now() + std::time::Duration::from_secs(60);
+    while !h.state().frames.is_cached(&h.state().shown_series(cid)) && std::time::Instant::now() < deadline {
+        std::thread::sleep(std::time::Duration::from_millis(5));
+    }
+    while h.state().playback.starting && std::time::Instant::now() < deadline {
+        h.step();
+        std::thread::sleep(std::time::Duration::from_millis(1));
+    }
+    assert!(!h.state().playback.starting, "the preview left its startup");
     assert!(h.state().audio.is_some(), "with its sound");
     assert_eq!(audio_frame(&h, cid), Some(0), "from the first frame");
+    let (started, stopped) = {
+        let d = fake.0.lock().unwrap();
+        (d.0.is_some(), d.1)
+    };
+    assert!(started && !stopped, "the device started and kept playing");
 }
 
 /// A first frame that takes longer than STARTUP_WAIT to be cached (a heavy comp, a busy machine;