diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index 72016cc5..541081af 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -1238,8 +1238,16 @@ impl EffectcraftApp {
         }
         let step = plan.map_or(1, |p| p.step);
         // Never further ahead than the RAM preview holds: those frames would evict the ones the
-        // preview shows first, or be evicted before it gets there (#595).
-        let window = (ahead * 3).min(self.frames_that_fit(&c, scale) - 1);
+        // preview shows first, or be evicted before it gets there (#595). A playing preview
+        // looks that far for the next uncached frames: after a loop the frames just ahead are
+        // cached, and a shorter window left the workers idle until the playhead came near the
+        // end of the cached run.
+        let fit_ahead = self.frames_that_fit(&c, scale) - 1;
+        let window = match plan.filter(|_| self.playback.playing) {
+            Some(p) => fit_ahead.min((p.end - p.start) / p.step.max(1) + 1),
+            None => (ahead * 3).min(fit_ahead),
+        };
+        let cached = self.frames.cached_frames(&series);
         for k in 0..window {
             if queued >= ahead as usize || !video {
                 break;
@@ -1257,7 +1265,7 @@ impl EffectcraftApp {
                     f
                 }
             };
-            if !self.frames.is_cached(&FrameKey { frame: f, ..series }) {
+            if cached.binary_search(&f).is_err() {
                 self.request_frame(cid, f, scale);
                 queued += 1;
             }
diff --git a/crates/ui-egui/tests/ui_preview_align.rs b/crates/ui-egui/tests/ui_preview_align.rs
index e9b8d6c4..2b2ba1b4 100644
--- a/crates/ui-egui/tests/ui_preview_align.rs
+++ b/crates/ui-egui/tests/ui_preview_align.rs
@@ -259,6 +259,45 @@ fn cache_frames_when_idle_fills_the_work_area() {
     assert_eq!(n, 240, "the work area is cached while idle");
 }
 
+/// A preview that plays into frames it already cached (after a loop) keeps rendering the next
+/// uncached ones: the prefetch window used to end within the cached run, and the workers stood
+/// idle until the playhead came near its end.
+#[test]
+fn playback_prefetches_past_the_cached_frames_ahead() {
+    let mut h = harness();
+    let cid = h.state().session.active_comp_id().unwrap();
+    let series = h.state().shown_series(cid);
+    let scale = series.scale as f64 / 1000.0;
+    // The first 120 frames (5 s) are cached: more than the widest old window (16 workers × 6).
+    for f in 0..120 {
+        h.state().request_frame(cid, f, scale);
+    }
+    for _ in 0..4000 {
+        if h.state().frames.cached_frames(&series).len() >= 120 && h.state().frames.inflight() == 0 {
+            break;
+        }
+        std::thread::sleep(std::time::Duration::from_millis(2));
+    }
+    assert_eq!(h.state().frames.cached_frames(&series), (0..120).collect::<Vec<_>>());
+    h.state_mut().session.set_time(Tick::ZERO);
+    invoke(&mut h, "playback.toggle", json!({}));
+    h.run_steps(2);
+    assert!(h.state().playback.playing);
+    let cur = h.state().session.time();
+    assert!(cur < Tick::from_seconds_f64(1.0), "{cur:?}");
+    for _ in 0..4000 {
+        if h.state().frames.cached_frames(&series).iter().any(|&f| f >= 120) {
+            break;
+        }
+        h.step();
+        std::thread::sleep(std::time::Duration::from_millis(1));
+        if h.state().session.time() >= Tick::from_seconds_f64(1.0) {
+            break;
+        }
+    }
+    assert!(h.state().frames.cached_frames(&series).iter().any(|&f| f >= 120), "frames past the cached run render while it plays the first second");
+}
+
 #[test]
 fn cache_frames_when_idle_survives_an_inverted_work_area() {
     let mut h = harness();