show entry in collection when leaving viewer

lib/model/highlight.dart

@@ -6,17 +6,22 @@ class HighlightInfo extends ChangeNotifier {
   final EventBus eventBus = EventBus();
 
   void trackItem<T>(
-    T item, {
+    T? item, {
+    TrackPredicate? predicate,
     Alignment? alignment,
     bool? animate,
     Object? highlightItem,
-  }) =>
+  }) {
+    if (item != null) {
       eventBus.fire(TrackEvent<T>(
         item,
+        predicate ?? (_) => true,
         alignment ?? Alignment.center,
         animate ?? true,
         highlightItem,
       ));
+    }
+  }
 
   Object? _item;
 
@@ -43,14 +48,20 @@ class HighlightInfo extends ChangeNotifier {
 @immutable
 class TrackEvent<T> {
   final T item;
+  final TrackPredicate predicate;
   final Alignment alignment;
   final bool animate;
   final Object? highlightItem;
 
   const TrackEvent(
     this.item,
+    this.predicate,
     this.alignment,
     this.animate,
     this.highlightItem,
   );
 }
+
+// `itemVisibility`: percent of the item tracked already visible in viewport
+// return whether to proceed with tracking
+typedef TrackPredicate = bool Function(double itemVisibility);

lib/widgets/common/grid/item_tracker.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:math';
 
 import 'package:aves/model/highlight.dart';
 import 'package:aves/theme/durations.dart';
@@ -36,12 +37,7 @@ mixin GridItemTrackerMixin<T, U extends StatefulWidget> on State<U>, WidgetsBind
   void initState() {
     super.initState();
     final highlightInfo = context.read<HighlightInfo>();
-    _subscriptions.add(highlightInfo.eventBus.on<TrackEvent<T>>().listen((e) => _trackItem(
-          e.item,
-          alignment: e.alignment,
-          animate: e.animate,
-          highlightItem: e.highlightItem,
-        )));
+    _subscriptions.add(highlightInfo.eventBus.on<TrackEvent<T>>().listen(_trackItem));
     _lastOrientation = _windowOrientation;
     WidgetsBinding.instance!.addObserver(this);
     _saveLayoutMetrics();
@@ -66,22 +62,21 @@ mixin GridItemTrackerMixin<T, U extends StatefulWidget> on State<U>, WidgetsBind
   // `Scrollable.ensureVisible` only works on already rendered objects
   // `RenderViewport.showOnScreen` can find any `RenderSliver`, but not always a `RenderMetadata`
   // `RenderViewport.scrollOffsetOf` is a good alternative
-  Future<void> _trackItem(
-    T item, {
-    required Alignment alignment,
-    required bool animate,
-    required Object? highlightItem,
-  }) async {
+  Future<void> _trackItem(TrackEvent event) async {
     final sectionedListLayout = context.read<SectionedListLayout<T>>();
-    final tileRect = sectionedListLayout.getTileRect(item);
+    final tileRect = sectionedListLayout.getTileRect(event.item);
     if (tileRect == null) return;
 
+    final viewportRect = Rect.fromLTWH(0, scrollController.offset, scrollableSize.width, scrollableSize.height);
+    final itemVisibility = max(0, tileRect.intersect(viewportRect).height) / tileRect.height;
+    if (!event.predicate(itemVisibility)) return;
+
     // most of the time the app bar will be scrolled away after scaling,
     // so we compensate for it to center the focal point thumbnail
     final appBarHeight = appBarHeightNotifier.value;
-    final scrollOffset = appBarHeight + tileRect.top + (tileRect.height - scrollableSize.height) * ((alignment.y + 1) / 2);
+    final scrollOffset = appBarHeight + tileRect.top + (tileRect.height - scrollableSize.height) * ((event.alignment.y + 1) / 2);
 
-    if (animate) {
+    if (event.animate) {
       if (scrollOffset > 0) {
         await scrollController.animateTo(
           scrollOffset,
@@ -95,6 +90,7 @@ mixin GridItemTrackerMixin<T, U extends StatefulWidget> on State<U>, WidgetsBind
       await Future.delayed(Durations.highlightJumpDelay);
     }
 
+    final highlightItem = event.highlightItem;
     if (highlightItem != null) {
       context.read<HighlightInfo>().set(highlightItem);
     }

lib/widgets/viewer/entry_viewer_stack.dart

@@ -3,6 +3,7 @@ import 'dart:math';
 import 'package:aves/model/actions/entry_actions.dart';
 import 'package:aves/model/entry.dart';
 import 'package:aves/model/filters/filters.dart';
+import 'package:aves/model/highlight.dart';
 import 'package:aves/model/multipage.dart';
 import 'package:aves/model/settings/enums.dart';
 import 'package:aves/model/settings/settings.dart';
@@ -62,6 +63,7 @@ class _EntryViewerStackState extends State<EntryViewerStack> with SingleTickerPr
   late EntryActionDelegate _actionDelegate;
   final List<Tuple2<String, ValueNotifier<ViewState>>> _viewStateNotifiers = [];
   final ValueNotifier<HeroInfo?> _heroInfoNotifier = ValueNotifier(null);
+  bool _isEntryTracked = true;
 
   CollectionLens? get collection => widget.collection;
 
@@ -166,6 +168,7 @@ class _EntryViewerStackState extends State<EntryViewerStack> with SingleTickerPr
           // back from info to image
           _goToVerticalPage(imagePage);
         } else {
+          if (!_isEntryTracked) _trackEntry();
           _popVisual();
         }
         return SynchronousFuture(false);
@@ -370,6 +373,9 @@ class _EntryViewerStackState extends State<EntryViewerStack> with SingleTickerPr
   }
 
   void _onVerticalPageControllerChange() {
+    if (!_isEntryTracked && _verticalPager.page?.floor() == transitionPage) {
+      _trackEntry();
+    }
     _verticalScrollNotifier.notifyListeners();
   }
 
@@ -451,6 +457,7 @@ class _EntryViewerStackState extends State<EntryViewerStack> with SingleTickerPr
     final newEntry = _currentHorizontalPage < entries.length ? entries[_currentHorizontalPage] : null;
     if (_entryNotifier.value == newEntry) return;
     _entryNotifier.value = newEntry;
+    _isEntryTracked = false;
     await _pauseVideoControllers();
     await _initEntryControllers();
   }
@@ -478,6 +485,20 @@ class _EntryViewerStackState extends State<EntryViewerStack> with SingleTickerPr
     }
   }
 
+  // track item when returning to collection,
+  // if they are not fully visible already
+  void _trackEntry() {
+    _isEntryTracked = true;
+    final entry = _entryNotifier.value;
+    if (entry != null && hasCollection) {
+      context.read<HighlightInfo>().trackItem(
+            entry,
+            predicate: (v) => v < 1,
+            animate: false,
+          );
+    }
+  }
+
   void _onLeave() {
     _showSystemUI();
     if (settings.keepScreenOn == KeepScreenOn.viewerOnly) {