fix(canvas): release closed panels' thumbnails while a visible page renders (#6227)

crates/ui-egui/src/canvas/cache.rs

@@ -458,7 +458,12 @@ impl DocView {
                 && !self.errors.contains_key(&r.page)
         });
         if visible_mains_pending {
-            // Caches are kept; demand is re-evaluated next frame once pixels arrive.
+            // Caches are kept; demand is re-evaluated next frame once pixels arrive. Thumbnails
+            // nothing shows any more (their panel closed) go now, though: no consumer is waiting.
+            if self.thumb_demand.is_empty() {
+                self.thumbs.clear();
+                self.stale_thumbs.clear();
+            }
             return queue;
         }
         let thumbs = self.thumbnail_requests(info, ppp);

crates/ui-egui/src/canvas/tests.rs

@@ -1428,6 +1428,64 @@ fn thumbnails_release_on_scroll_panel_close_and_request_again_after_eviction() {
     assert!(v.prepare_render_queue(&info, 1.0).is_empty(), "closing consumers cancels thumbnail work");
 }
 
+#[test]
+fn closing_the_last_thumbnail_consumer_releases_thumbnails_while_a_visible_page_renders() {
+    let ctx = egui::Context::default();
+    let info = thumbnail_info(1);
+    let mut v = DocView::new(DocId(1), &info, ViewDefaults::default());
+    v.need_thumbnail(0, true);
+    let first = v.prepare_render_queue(&info, 1.0);
+    v.last_queue = first.clone();
+    assert_eq!(v.receive_result(&ctx, solid_result(first[0], 132, 171)), 132 * 171 * 4);
+    let main = RenderRequest { scale: 1.0, tag: 1000, ..Default::default() };
+    v.begin_render_frame();
+    v.need_thumbnail(0, true);
+    v.frame_queue = vec![main];
+    v.frame_visible.insert(0);
+    assert_eq!(v.prepare_render_queue(&info, 1.0), vec![main]);
+    assert!(!v.thumbs.is_empty(), "a thumbnail still shown waits for the page");
+    v.begin_render_frame();
+    v.frame_queue = vec![main];
+    v.frame_visible.insert(0);
+    v.stale_thumbs.insert(0);
+    assert_eq!(v.prepare_render_queue(&info, 1.0), vec![main]);
+    assert!(v.thumbs.is_empty(), "nothing shows it any more, whatever the page is doing");
+    assert!(v.stale_thumbs.is_empty(), "nor is it remembered as out of date");
+    // Shown again once the page is drawn: the thumbnail is asked for afresh.
+    v.last_queue = vec![main];
+    assert!(v.receive_result(&ctx, solid_result(main, 100, 130)) > 0);
+    v.begin_render_frame();
+    v.frame_visible.insert(0);
+    v.need_thumbnail(0, true);
+    let queue = v.prepare_render_queue(&info, 1.0);
+    assert!(queue.iter().any(|r| r.page == 0 && r.tag & THUMB_TAG != 0), "the thumbnail is asked for again: {queue:?}");
+}
+
+/// While anything still shows a thumbnail, the thumbnails all wait for the visible page, as before:
+/// only the last consumer closing lets them go early.
+#[test]
+fn a_thumbnail_still_shown_keeps_the_others_while_a_visible_page_renders() {
+    let ctx = egui::Context::default();
+    // Pages 0 and 3 are the same size.
+    let info = thumbnail_info(4);
+    let mut v = DocView::new(DocId(1), &info, ViewDefaults::default());
+    v.need_thumbnail(0, true);
+    v.need_thumbnail(3, true);
+    let first = v.prepare_render_queue(&info, 1.0);
+    v.last_queue = first.clone();
+    for r in &first {
+        assert!(v.receive_result(&ctx, solid_result(*r, 132, 171)) > 0, "thumbnail of page {} taken", r.page);
+    }
+    assert_eq!(v.thumbs.len(), 2);
+    let main = RenderRequest { scale: 1.0, tag: 1000, ..Default::default() };
+    v.begin_render_frame();
+    v.need_thumbnail(0, true);
+    v.frame_queue = vec![main];
+    v.frame_visible.insert(0);
+    assert_eq!(v.prepare_render_queue(&info, 1.0), vec![main]);
+    assert!(v.thumbs.contains_key(&0) && v.thumbs.contains_key(&3), "both wait for the page");
+}
+
 #[test]
 fn stale_results_are_rejected_before_conversion_after_dpi_invalidation_and_suspend() {
     let ctx = egui::Context::default();