collection: snap to header/row when fast scrolling long content

lib/widgets/collection/collection_grid.dart

@@ -45,6 +45,7 @@ import 'package:aves/widgets/common/thumbnail/notifications.dart';
 import 'package:aves/widgets/common/tile_extent_controller.dart';
 import 'package:aves/widgets/navigation/nav_bar/nav_bar.dart';
 import 'package:aves/widgets/viewer/entry_viewer_page.dart';
+import 'package:collection/collection.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_staggered_animations/flutter_staggered_animations.dart';
@@ -500,6 +501,8 @@ class _CollectionScrollViewState extends State<_CollectionScrollView> with Widge
                 return Selector<SectionedListLayout<AvesEntry>, List<SectionLayout>>(
                   selector: (context, layout) => layout.sectionLayouts,
                   builder: (context, sectionLayouts, child) {
+                    final scrollController = widget.scrollController;
+                    final offsetIncrementSnapThreshold = context.select<TileExtentController, double>((v) => (v.extentNotifier.value + v.spacing) / 4);
                     return DraggableScrollbar(
                       backgroundColor: Colors.white,
                       scrollThumbSize: Size(avesScrollThumbWidth, avesScrollThumbHeight),
@@ -507,7 +510,23 @@ class _CollectionScrollViewState extends State<_CollectionScrollView> with Widge
                         height: avesScrollThumbHeight,
                         backgroundColor: Colors.white,
                       ),
-                      controller: widget.scrollController,
+                      controller: scrollController,
+                      dragOffsetSnapper: (scrollOffset, offsetIncrement) {
+                        if (offsetIncrement > offsetIncrementSnapThreshold && scrollOffset < scrollController.position.maxScrollExtent) {
+                          final section = sectionLayouts.firstWhereOrNull((section) => section.hasChildAtOffset(scrollOffset));
+                          if (section != null) {
+                            if (section.maxOffset - section.minOffset < scrollController.position.viewportDimension) {
+                              // snap to section header
+                              return section.minOffset;
+                            } else {
+                              // snap to content row
+                              final index = section.getMinChildIndexForScrollOffset(scrollOffset);
+                              return section.indexToLayoutOffset(index);
+                            }
+                          }
+                        }
+                        return scrollOffset;
+                      },
                       crumbsBuilder: () => _getCrumbs(sectionLayouts),
                       padding: EdgeInsets.only(
                         // padding to keep scroll thumb between app bar above and nav bar below

lib/widgets/common/basic/draggable_scrollbar/scrollbar.dart

@@ -59,6 +59,8 @@ class DraggableScrollbar extends StatefulWidget {
   /// The ScrollController for the BoxScrollView
   final ScrollController controller;
 
+  final double Function(double scrollOffset, double offsetIncrement)? dragOffsetSnapper;
+
   /// The view that will be scrolled with the scroll thumb
   final ScrollView child;
 
@@ -68,6 +70,7 @@ class DraggableScrollbar extends StatefulWidget {
     required this.scrollThumbSize,
     required this.scrollThumbBuilder,
     required this.controller,
+    this.dragOffsetSnapper,
     this.crumbsBuilder,
     this.padding = EdgeInsets.zero,
     this.scrollbarAnimationDuration = const Duration(milliseconds: 300),
@@ -114,6 +117,7 @@ class DraggableScrollbar extends StatefulWidget {
 class _DraggableScrollbarState extends State<DraggableScrollbar> with TickerProviderStateMixin {
   final ValueNotifier<double> _thumbOffsetNotifier = ValueNotifier(0), _viewOffsetNotifier = ValueNotifier(0);
   bool _isDragInProcess = false;
+  double _boundlessThumbOffset = 0, _offsetIncrement = 0;
   late Offset _longPressLastGlobalPosition;
 
   late AnimationController _thumbAnimationController;
@@ -281,6 +285,8 @@ class _DraggableScrollbarState extends State<DraggableScrollbar> with TickerProv
 
   void _onVerticalDragStart() {
     const DraggableScrollbarNotification(DraggableScrollbarEvent.dragStart).dispatch(context);
+    _boundlessThumbOffset = _thumbOffsetNotifier.value;
+    _offsetIncrement = 1 / thumbMaxScrollExtent * controller.position.maxScrollExtent;
     _labelAnimationController.forward();
     _fadeoutTimer?.cancel();
     _showThumb();
@@ -292,12 +298,14 @@ class _DraggableScrollbarState extends State<DraggableScrollbar> with TickerProv
     _showThumb();
     if (_isDragInProcess) {
       // thumb offset
-      _thumbOffsetNotifier.value = (_thumbOffsetNotifier.value + deltaY).clamp(thumbMinScrollExtent, thumbMaxScrollExtent);
+      _boundlessThumbOffset += deltaY;
+      _thumbOffsetNotifier.value = _boundlessThumbOffset.clamp(thumbMinScrollExtent, thumbMaxScrollExtent);
 
       // scroll offset
       final min = controller.position.minScrollExtent;
       final max = controller.position.maxScrollExtent;
-      controller.jumpTo((_thumbOffsetNotifier.value / thumbMaxScrollExtent * max).clamp(min, max));
+      final scrollOffset = _thumbOffsetNotifier.value / thumbMaxScrollExtent * max;
+      controller.jumpTo((widget.dragOffsetSnapper?.call(scrollOffset, _offsetIncrement) ?? scrollOffset).clamp(min, max));
     }
   }