fix(mobile): keep drag select going while the grid auto scrolls

mobile/lib/domain/services/timeline.service.dart

@@ -182,6 +182,18 @@ class TimelineService {
     return _buffer.slice(start, start + count);
   }
 
+  /// Reads a range without disturbing the buffer; queries the source if it isn't resident.
+  Future<List<BaseAsset>> getAssetsRange(int index, int count) async {
+    if (index < 0 || count <= 0 || index >= _totalAssets) {
+      return const [];
+    }
+    final clamped = math.min(count, _totalAssets - index);
+    if (hasRange(index, clamped)) {
+      return getAssets(index, clamped);
+    }
+    return _assetSource(index, clamped);
+  }
+
   // Preload assets around the given index for asset viewer
   Future<void> preloadAssets(int index) => _mutex.run(() => _loadAssets(index, math.min(5, _totalAssets - index)));
 

mobile/lib/presentation/widgets/timeline/drag_selection_controller.dart

@@ -0,0 +1,175 @@
+import 'dart:collection';
+
+import 'package:immich_mobile/domain/models/asset/base_asset.model.dart';
+
+// Tracks the [anchor..current] range selected by a drag. The in-buffer part of
+// each tick is selected synchronously so it follows the finger without racing;
+// the rare beyond-buffer part is read async (applied only if still in range) and
+// [end] reads whatever is still missing so the final range always lands.
+class DragSelectionController {
+  DragSelectionController({required this.getAssetSafe, required this.getAssetsRange, required this.onChange});
+
+  final BaseAsset? Function(int index) getAssetSafe;
+  final Future<List<BaseAsset>> Function(int index, int count) getAssetsRange;
+  final void Function(Set<BaseAsset> select, Set<BaseAsset> deselect) onChange;
+
+  final HashMap<int, BaseAsset> _selected = HashMap();
+  // Indices the buffer didn't hold yet (the edge outran the async buffer-load on a
+  // fast scroll); read in by _extendPending and guaranteed by end().
+  final Set<int> _pending = {};
+  int? _anchor;
+  int? _lo;
+  int? _hi;
+  bool _disposed = false;
+
+  // Call before starting a new drag so a previous drag's in-flight read can't
+  // leak into the new selection.
+  void dispose() => _disposed = true;
+
+  void _emit(Set<BaseAsset> select, Set<BaseAsset> deselect) {
+    if (_disposed) {
+      return;
+    }
+    onChange(select, deselect);
+  }
+
+  void start(int anchor) {
+    _selected.clear();
+    _pending.clear();
+    _anchor = anchor;
+    _lo = anchor;
+    _hi = anchor;
+    _select(anchor);
+    _extendPending();
+  }
+
+  void enter(int current) {
+    final anchor = _anchor;
+    if (anchor == null || _lo == null || _hi == null) {
+      return;
+    }
+    final ns = current < anchor ? current : anchor;
+    final ne = current < anchor ? anchor : current;
+    final ps = _lo!;
+    final pe = _hi!;
+    if (ns == ps && ne == pe) {
+      return;
+    }
+
+    final toSelect = <BaseAsset>{};
+    final toDeselect = <BaseAsset>{};
+
+    _forEach(ps, ns - 1, (k) => _removeIndex(k, toDeselect));
+    _forEach(ne + 1, pe, (k) => _removeIndex(k, toDeselect));
+    _forEach(ns, ps - 1, (k) => _addIndex(k, toSelect));
+    _forEach(pe + 1, ne, (k) => _addIndex(k, toSelect));
+
+    _lo = ns;
+    _hi = ne;
+
+    if (toSelect.isNotEmpty || toDeselect.isNotEmpty) {
+      _emit(toSelect, toDeselect);
+    }
+    _extendPending();
+  }
+
+  Future<void> end() async {
+    final lo = _lo;
+    final hi = _hi;
+    if (lo == null || hi == null) {
+      return;
+    }
+    final missing = <int>[];
+    for (var k = lo; k <= hi; k++) {
+      if (!_selected.containsKey(k)) {
+        missing.add(k);
+      }
+    }
+    if (missing.isEmpty) {
+      return;
+    }
+    final from = missing.first;
+    final assets = await getAssetsRange(from, missing.last - from + 1);
+    final missingSet = missing.toSet();
+    final toSelect = <BaseAsset>{};
+    for (var i = 0; i < assets.length; i++) {
+      final idx = from + i;
+      if (missingSet.contains(idx) && !_selected.containsKey(idx)) {
+        _selected[idx] = assets[i];
+        _pending.remove(idx);
+        toSelect.add(assets[i]);
+      }
+    }
+    if (toSelect.isNotEmpty) {
+      _emit(toSelect, const {});
+    }
+  }
+
+  void _select(int index) {
+    final asset = getAssetSafe(index);
+    if (asset != null) {
+      _selected[index] = asset;
+      _pending.remove(index);
+      _emit({asset}, const {});
+    } else {
+      _pending.add(index);
+    }
+  }
+
+  void _addIndex(int index, Set<BaseAsset> toSelect) {
+    if (_selected.containsKey(index)) {
+      return;
+    }
+    final asset = getAssetSafe(index);
+    if (asset != null) {
+      _selected[index] = asset;
+      _pending.remove(index);
+      toSelect.add(asset);
+    } else {
+      _pending.add(index);
+    }
+  }
+
+  void _removeIndex(int index, Set<BaseAsset> toDeselect) {
+    _pending.remove(index);
+    final asset = _selected.remove(index);
+    if (asset != null) {
+      toDeselect.add(asset);
+    }
+  }
+
+  Future<void> _extendPending() async {
+    if (_pending.isEmpty) {
+      return;
+    }
+    var from = _pending.first;
+    var to = _pending.first;
+    for (final k in _pending) {
+      if (k < from) {
+        from = k;
+      }
+      if (k > to) {
+        to = k;
+      }
+    }
+    final assets = await getAssetsRange(from, to - from + 1);
+    final toSelect = <BaseAsset>{};
+    for (var i = 0; i < assets.length; i++) {
+      final idx = from + i;
+      if (_pending.contains(idx) && _lo != null && idx >= _lo! && idx <= _hi!) {
+        _selected[idx] = assets[i];
+        toSelect.add(assets[i]);
+      }
+    }
+    _pending.removeWhere((idx) => _selected.containsKey(idx));
+    if (toSelect.isNotEmpty) {
+      _emit(toSelect, const {});
+    }
+  }
+
+  void _forEach(int lo, int hi, void Function(int) fn) {
+    for (var k = lo; k <= hi; k++) {
+      fn(k);
+    }
+  }
+}

mobile/lib/presentation/widgets/timeline/timeline.widget.dart

@@ -1,5 +1,4 @@
 import 'dart:async';
-import 'dart:collection';
 import 'dart:math' as math;
 
 import 'package:collection/collection.dart';
@@ -8,7 +7,6 @@ import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:hooks_riverpod/hooks_riverpod.dart';
-import 'package:immich_mobile/domain/models/asset/base_asset.model.dart';
 import 'package:immich_mobile/domain/models/events.model.dart';
 import 'package:immich_mobile/domain/models/timeline.model.dart';
 import 'package:immich_mobile/domain/utils/event_stream.dart';
@@ -20,6 +18,7 @@ import 'package:immich_mobile/presentation/widgets/timeline/constants.dart';
 import 'package:immich_mobile/presentation/widgets/timeline/scrubber.widget.dart';
 import 'package:immich_mobile/presentation/widgets/timeline/segment.model.dart';
 import 'package:immich_mobile/presentation/widgets/timeline/timeline.state.dart';
+import 'package:immich_mobile/presentation/widgets/timeline/drag_selection_controller.dart';
 import 'package:immich_mobile/presentation/widgets/timeline/timeline_drag_region.dart';
 import 'package:immich_mobile/providers/infrastructure/readonly_mode.provider.dart';
 import 'package:immich_mobile/providers/infrastructure/settings.provider.dart';
@@ -29,6 +28,27 @@ import 'package:immich_mobile/widgets/common/immich_sliver_app_bar.dart';
 import 'package:immich_mobile/widgets/common/mesmerizing_sliver_app_bar.dart';
 import 'package:immich_mobile/widgets/common/selection_sliver_app_bar.dart';
 
+// First asset index of the row shown at [offset]. Pure for testing.
+@visibleForTesting
+int? assetIndexAtOffset(
+  List<Segment> segments,
+  double offset, {
+  required int columnCount,
+  required double maxScrollExtent,
+}) {
+  final clamped = offset.clamp(0.0, maxScrollExtent);
+  final segment = segments.findByOffset(clamped) ?? segments.lastOrNull;
+  if (segment == null) {
+    return null;
+  }
+  final rowIndex = segment.getMinChildIndexForScrollOffset(clamped);
+  if (rowIndex > segment.firstIndex) {
+    final rowIndexInSegment = rowIndex - (segment.firstIndex + 1);
+    return segment.firstAssetIndex + rowIndexInSegment * columnCount;
+  }
+  return segment.firstAssetIndex;
+}
+
 class Timeline extends StatelessWidget {
   const Timeline({
     super.key,
@@ -140,9 +160,10 @@ class _SliverTimelineState extends ConsumerState<_SliverTimeline> {
   StreamSubscription? _eventSubscription;
 
   // Drag selection state
+  static const _autoScrollStep = 175.0;
+  static const _autoScrollDuration = Duration(milliseconds: 125);
   bool _dragging = false;
-  TimelineAssetIndex? _dragAnchorIndex;
-  final Set<BaseAsset> _draggedAssets = HashSet();
+  DragSelectionController? _dragController;
   ScrollPhysics? _scrollPhysics;
 
   int _perRow = 4;
@@ -226,26 +247,18 @@ class _SliverTimelineState extends ConsumerState<_SliverTimeline> {
     EventStream.shared.emit(MultiSelectToggleEvent(isEnabled));
   }
 
-  int? _getCurrentAssetIndex(List<Segment> segments) {
-    final currentOffset = _scrollController.offset.clamp(0.0, _scrollController.position.maxScrollExtent);
-    final segment = segments.findByOffset(currentOffset) ?? segments.lastOrNull;
-    int? targetAssetIndex;
-    if (segment != null) {
-      final rowIndex = segment.getMinChildIndexForScrollOffset(currentOffset);
-      if (rowIndex > segment.firstIndex) {
-        final rowIndexInSegment = rowIndex - (segment.firstIndex + 1);
-        final assetsPerRow = ref.read(timelineArgsProvider).columnCount;
-        final assetIndexInSegment = rowIndexInSegment * assetsPerRow;
-        targetAssetIndex = segment.firstAssetIndex + assetIndexInSegment;
-      } else {
-        targetAssetIndex = segment.firstAssetIndex;
-      }
-    }
-    return targetAssetIndex;
-  }
+  int? _getCurrentAssetIndex(List<Segment> segments) => _assetIndexAtOffset(segments, _scrollController.offset);
+
+  int? _assetIndexAtOffset(List<Segment> segments, double offset) => assetIndexAtOffset(
+    segments,
+    offset,
+    columnCount: ref.read(timelineArgsProvider).columnCount,
+    maxScrollExtent: _scrollController.position.maxScrollExtent,
+  );
 
   @override
   void dispose() {
+    _dragController?.dispose();
     _scrollController.dispose();
     _eventSubscription?.cancel();
     super.dispose();
@@ -295,9 +308,21 @@ class _SliverTimelineState extends ConsumerState<_SliverTimeline> {
 
   // Drag selection methods
   void _setDragStartIndex(TimelineAssetIndex index) {
+    // Stop the old drag's controller so its in-flight read can't leak into this one.
+    _dragController?.dispose();
+    final timelineService = ref.read(timelineServiceProvider);
+    _dragController = DragSelectionController(
+      getAssetSafe: timelineService.getAssetSafe,
+      getAssetsRange: timelineService.getAssetsRange,
+      onChange: (select, deselect) {
+        if (!mounted) {
+          return;
+        }
+        ref.read(multiSelectProvider.notifier).selectRange(select, deselect);
+      },
+    )..start(index.assetIndex);
     setState(() {
       _scrollPhysics = const ClampingScrollPhysics();
-      _dragAnchorIndex = index;
       _dragging = true;
     });
   }
@@ -313,8 +338,12 @@ class _SliverTimelineState extends ConsumerState<_SliverTimeline> {
     });
     setState(() {
       _dragging = false;
-      _draggedAssets.clear();
     });
+    // Apply the full final range even if a read is still in flight on lift.
+    final finishing = _dragController?.end();
+    if (finishing != null) {
+      unawaited(finishing);
+    }
     final timelineState = ref.read(timelineStateProvider.notifier);
     Future.delayed(const Duration(milliseconds: 300), () {
       timelineState.setScrolling(false);
@@ -322,42 +351,33 @@ class _SliverTimelineState extends ConsumerState<_SliverTimeline> {
   }
 
   void _dragScroll(ScrollDirection direction) {
-    _scrollController.animateTo(
-      _scrollController.offset + (direction == ScrollDirection.forward ? 175 : -175),
-      duration: const Duration(milliseconds: 125),
-      curve: Curves.easeOut,
-    );
+    final position = _scrollController.position;
+    final step = direction == ScrollDirection.forward ? _autoScrollStep : -_autoScrollStep;
+    final target = (_scrollController.offset + step).clamp(0.0, position.maxScrollExtent);
+    _scrollController.animateTo(target, duration: _autoScrollDuration, curve: Curves.easeOut);
+
+    // A held finger emits no move events, so extend the selection to the asset
+    // at the leading edge of the scroll instead.
+    final controller = _dragController;
+    if (controller == null) {
+      return;
+    }
+    final segments = ref.read(timelineSegmentProvider).valueOrNull;
+    if (segments == null) {
+      return;
+    }
+    final edgeOffset = direction == ScrollDirection.forward ? target + position.viewportDimension : target;
+    final edgeIndex = _assetIndexAtOffset(segments, edgeOffset);
+    if (edgeIndex != null) {
+      controller.enter(edgeIndex);
+    }
   }
 
   void _handleDragAssetEnter(TimelineAssetIndex index) {
-    if (_dragAnchorIndex == null || !_dragging) {
+    if (!_dragging) {
       return;
     }
-
-    final timelineService = ref.read(timelineServiceProvider);
-    final dragAnchorIndex = _dragAnchorIndex!;
-
-    // Calculate the range of assets to select
-    final startIndex = math.min(dragAnchorIndex.assetIndex, index.assetIndex);
-    final endIndex = math.max(dragAnchorIndex.assetIndex, index.assetIndex);
-    final count = endIndex - startIndex + 1;
-
-    // Load the assets in the range
-    if (timelineService.hasRange(startIndex, count)) {
-      final selectedAssets = timelineService.getAssets(startIndex, count);
-
-      // Clear previous drag selection and add new range
-      final multiSelectNotifier = ref.read(multiSelectProvider.notifier);
-      for (final asset in _draggedAssets) {
-        multiSelectNotifier.deselectAsset(asset);
-      }
-      _draggedAssets.clear();
-
-      for (final asset in selectedAssets) {
-        multiSelectNotifier.selectAsset(asset);
-        _draggedAssets.add(asset);
-      }
-    }
+    _dragController?.enter(index.assetIndex);
   }
 
   @override

mobile/lib/providers/timeline/multiselect.provider.dart

@@ -97,6 +97,15 @@ class MultiSelectNotifier extends Notifier<MultiSelectState> {
     }
   }
 
+  // Drops the previous drag range and adds the new one in a single update. The
+  // full-set copy per drag tick is the accepted cost of immutable state.
+  void selectRange(Set<BaseAsset> toSelect, Set<BaseAsset> toDeselect) {
+    final selectedAssets = state.selectedAssets.toSet()
+      ..removeAll(toDeselect)
+      ..addAll(toSelect);
+    state = state.copyWith(selectedAssets: selectedAssets);
+  }
+
   void reset() {
     state = const MultiSelectState(selectedAssets: {}, lockedSelectionAssets: {}, forceEnable: false);
   }

mobile/test/unit/presentation/widgets/timeline/asset_index_at_offset_test.dart

@@ -0,0 +1,60 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:immich_mobile/domain/models/timeline.model.dart';
+import 'package:immich_mobile/presentation/widgets/timeline/fixed/segment.model.dart';
+import 'package:immich_mobile/presentation/widgets/timeline/segment.model.dart';
+import 'package:immich_mobile/presentation/widgets/timeline/timeline.widget.dart';
+
+void main() {
+  // Two day-segments, 4 columns, 8 assets each (2 rows). header 50, tile 100, no spacing.
+  //   A: header@[0,50)  rows@50,150   assets 0..7    offset [0,250]
+  //   B: header@[250,300) rows@300,400 assets 8..15  offset [250,500]
+  const columnCount = 4;
+  const maxScrollExtent = 500.0;
+  final segments = <Segment>[
+    const FixedSegment(
+      firstIndex: 0,
+      lastIndex: 2,
+      startOffset: 0,
+      endOffset: 250,
+      firstAssetIndex: 0,
+      bucket: Bucket(assetCount: 8),
+      tileHeight: 100,
+      columnCount: columnCount,
+      headerExtent: 50,
+      spacing: 0,
+      header: HeaderType.day,
+    ),
+    const FixedSegment(
+      firstIndex: 3,
+      lastIndex: 5,
+      startOffset: 250,
+      endOffset: 500,
+      firstAssetIndex: 8,
+      bucket: Bucket(assetCount: 8),
+      tileHeight: 100,
+      columnCount: columnCount,
+      headerExtent: 50,
+      spacing: 0,
+      header: HeaderType.day,
+    ),
+  ];
+
+  int? at(double offset) =>
+      assetIndexAtOffset(segments, offset, columnCount: columnCount, maxScrollExtent: maxScrollExtent);
+
+  test('maps an offset to the first asset of the row shown there', () {
+    expect(at(0), 0); // top of segment A
+    expect(at(150), 4); // second row of A
+    expect(at(350), 8); // first row of B
+    expect(at(450), 12); // second row of B
+  });
+
+  test('clamps offsets outside the scroll range', () {
+    expect(at(-100), 0); // below the top -> first asset
+    expect(at(9999), at(maxScrollExtent)); // past the end -> same as the max offset
+  });
+
+  test('returns null for empty segments', () {
+    expect(assetIndexAtOffset(const [], 100, columnCount: columnCount, maxScrollExtent: maxScrollExtent), isNull);
+  });
+}

mobile/test/unit/presentation/widgets/timeline/drag_selection_controller_test.dart

@@ -0,0 +1,157 @@
+import 'dart:async';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:immich_mobile/domain/models/asset/base_asset.model.dart';
+import 'package:immich_mobile/presentation/widgets/timeline/drag_selection_controller.dart';
+
+import '../../../factories/remote_asset_factory.dart';
+
+void main() {
+  const total = 50;
+  late List<BaseAsset> all;
+  late Set<int> inBuffer; // indices getAssetSafe resolves synchronously
+  late List<Completer<List<BaseAsset>>> reads; // pending async reads, completed manually
+  late List<({int from, int count})> readArgs;
+  late Set<BaseAsset> selected;
+  late DragSelectionController sut;
+
+  Set<BaseAsset> range(int lo, int hi) => {for (var i = lo; i <= hi; i++) all[i]};
+
+  void completeRead(int i) {
+    final a = readArgs[i];
+    reads[i].complete([for (var k = a.from; k < a.from + a.count && k < total; k++) all[k]]);
+  }
+
+  Future<void> settle() => Future(() {});
+
+  setUp(() {
+    all = List.generate(total, (i) => RemoteAssetFactory.create(id: 'a${i.toString().padLeft(3, '0')}'));
+    inBuffer = {for (var i = 0; i < total; i++) i}; // default: everything buffered (sync)
+    reads = [];
+    readArgs = [];
+    selected = {};
+    sut = DragSelectionController(
+      getAssetSafe: (i) => inBuffer.contains(i) ? all[i] : null,
+      getAssetsRange: (from, count) {
+        final c = Completer<List<BaseAsset>>();
+        reads.add(c);
+        readArgs.add((from: from, count: count));
+        return c.future;
+      },
+      onChange: (select, deselect) {
+        selected
+          ..addAll(select)
+          ..removeAll(deselect);
+      },
+    );
+  });
+
+  group('live selection (all in buffer, synchronous)', () {
+    test('dragging down selects the whole range as it grows', () {
+      sut.start(2);
+      sut.enter(6);
+      sut.enter(12);
+      expect(selected, range(2, 12));
+      expect(reads, isEmpty, reason: 'everything buffered -> no async read');
+    });
+
+    test('dragging back up toward the anchor deselects the shrunk tail', () {
+      sut.start(2);
+      sut.enter(12);
+      expect(selected, range(2, 12));
+      sut.enter(6); // reverse
+      expect(selected, range(2, 6));
+      sut.enter(3); // reverse more
+      expect(selected, range(2, 3));
+    });
+
+    test('dragging past the anchor flips the range', () {
+      sut.start(10);
+      sut.enter(14);
+      expect(selected, range(10, 14));
+      sut.enter(7); // crosses the anchor
+      expect(selected, range(7, 10));
+    });
+  });
+
+  group('beyond-buffer (async)', () {
+    setUp(() => inBuffer = {}); // nothing buffered -> every tile needs an async read
+
+    test('drag-end fills the full range even if every live read is still in flight', () async {
+      sut.start(0);
+      sut.enter(20);
+      // simulate the real-rate race: none of the in-drag reads have completed
+      expect(selected, isEmpty);
+
+      final ending = sut.end();
+      // end() issues its own read for the missing range; complete it
+      completeRead(reads.length - 1);
+      await ending;
+
+      expect(selected, range(0, 20), reason: 'final range must always apply on drag-end');
+    });
+
+    test('out-of-order live read completions never corrupt the selection', () async {
+      sut.start(0); // issues read for [0,1]
+      sut.enter(10); // issues read for [0,11]
+      sut.enter(20); // issues read for [0,21]
+      expect(reads.length, 3);
+
+      // complete newest first, then older ones (out of order)
+      completeRead(2);
+      await settle();
+      completeRead(1);
+      await settle();
+      completeRead(0);
+      await settle();
+
+      expect(selected, range(0, 20));
+
+      final ending = sut.end();
+      await ending; // nothing missing -> no extra read
+      expect(selected, range(0, 20));
+    });
+
+    test('a disposed controller never emits when its in-flight read resolves', () async {
+      sut.start(0); // read [0,1] (pending, in flight)
+      sut.enter(20); // read [0,21] (pending, in flight)
+      expect(selected, isEmpty);
+
+      // a new drag starts -> the old controller is disposed
+      final ending = sut.end(); // issues end()'s fill read
+      sut.dispose();
+
+      // every in-flight read for the old controller now resolves
+      for (var i = 0; i < reads.length; i++) {
+        if (!reads[i].isCompleted) {
+          completeRead(i);
+        }
+      }
+      await ending;
+      await settle();
+
+      expect(selected, isEmpty, reason: 'a disposed controller must not leak into the new selection');
+    });
+
+    test('a late read for tiles dragged back out of range is ignored', () async {
+      sut.start(0); // read [0,1]
+      sut.enter(20); // read [0,21]
+      sut.enter(5); // shrink back; read [0,6]
+      // complete the stale wide read AFTER the shrink
+      completeRead(1); // [0,21]
+      await settle();
+      // indices 6..20 left the range -> must not be selected
+      expect(selected.intersection(range(6, 20)), isEmpty);
+
+      final ending = sut.end();
+      // complete any read end() issued for the (now smaller) missing range
+      for (var i = 0; i < reads.length; i++) {
+        if (!reads[i].isCompleted) {
+          completeRead(i);
+        }
+      }
+      await ending;
+      expect(selected, range(0, 5));
+    });
+  });
+}

mobile/test/unit/services/timeline_service_test.dart

@@ -0,0 +1,100 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:immich_mobile/constants/constants.dart';
+import 'package:immich_mobile/domain/models/asset/base_asset.model.dart';
+import 'package:immich_mobile/domain/models/timeline.model.dart';
+import 'package:immich_mobile/domain/services/timeline.service.dart';
+
+import '../factories/remote_asset_factory.dart';
+
+void main() {
+  // total must exceed the sliding buffer so it cannot hold the whole library at once
+  const total = 2000;
+  late List<BaseAsset> all;
+  late TimelineService sut;
+
+  TimelineService buildService() {
+    all = List.generate(total, (i) => RemoteAssetFactory.create(id: 'a${i.toString().padLeft(5, '0')}'));
+    return TimelineService((
+      assetSource: (index, count) async {
+        final end = (index + count) > total ? total : index + count;
+        return all.sublist(index, end);
+      },
+      bucketSource: () => Stream.value([const Bucket(assetCount: total)]),
+      origin: TimelineOrigin.main,
+    ));
+  }
+
+  Future<void> settle() => Future.delayed(const Duration(milliseconds: 10));
+
+  setUp(() async {
+    sut = buildService();
+    await settle(); // let the bucket subscription load the first batch and set totalAssets
+  });
+
+  tearDown(() async {
+    await sut.dispose();
+  });
+
+  test('buffer holds the first batch but not the whole library', () {
+    expect(sut.totalAssets, total);
+    expect(sut.hasRange(0, kTimelineAssetLoadBatchSize), isTrue);
+    expect(sut.hasRange(0, total), isFalse);
+  });
+
+  // #27118 / #20855 mechanism: drag-selecting from a low anchor while the grid
+  // auto-scrolls down slides the buffer forward to follow the finger. once the
+  // buffer offset passes the anchor, hasRange(anchor, ...) is false and
+  // _handleDragAssetEnter silently stops extending the selection.
+  test('anchor drops out of the buffer after the grid scrolls down', () async {
+    const anchor = 5;
+    expect(sut.hasRange(anchor, 100), isTrue);
+
+    // the grid loads a far-down range as it auto-scrolls during the drag
+    await sut.loadAssets(1500, 1);
+
+    const current = 1500;
+    const count = current - anchor + 1;
+    expect(sut.hasRange(anchor, 1), isFalse, reason: 'anchor is now below the buffer offset');
+    expect(sut.hasRange(anchor, count), isFalse, reason: 'the full drag range is no longer resident');
+    expect(() => sut.getAssets(anchor, count), throwsRangeError);
+  });
+
+  // the fix: getAssetsRange returns the whole drag range regardless of the
+  // buffer position, so the selection keeps extending while scrolling.
+  group('getAssetsRange', () {
+    test('returns a buffered range', () async {
+      final assets = await sut.getAssetsRange(0, 50);
+      expect(assets.length, 50);
+      expect(assets.first, all[0]);
+      expect(assets.last, all[49]);
+    });
+
+    test('returns a range wider than the buffer', () async {
+      final assets = await sut.getAssetsRange(0, total);
+      expect(assets.length, total);
+      expect(assets.first, all[0]);
+      expect(assets.last, all[total - 1]);
+    });
+
+    test('returns the anchor range after the buffer scrolled past the anchor', () async {
+      const anchor = 5;
+      await sut.loadAssets(1500, 1); // buffer slides forward, dropping the anchor
+      expect(sut.hasRange(anchor, 1), isFalse);
+
+      const current = 1500;
+      const count = current - anchor + 1;
+      final assets = await sut.getAssetsRange(anchor, count);
+      expect(assets.length, count);
+      expect(assets.first, all[anchor]);
+      expect(assets.last, all[current]);
+    });
+
+    test('clamps a range that runs past the end and ignores invalid input', () async {
+      final tail = await sut.getAssetsRange(total - 10, 100);
+      expect(tail.length, 10);
+      expect(await sut.getAssetsRange(-1, 10), isEmpty);
+      expect(await sut.getAssetsRange(0, 0), isEmpty);
+      expect(await sut.getAssetsRange(total, 10), isEmpty);
+    });
+  });
+}