collection: sloppy scroll physics to improve scale gesture recognition

lib/widgets/album/thumbnail_collection.dart

@@ -12,6 +12,7 @@ import 'package:aves/widgets/album/grid/scaling.dart';
 import 'package:aves/widgets/album/grid/tile_extent_manager.dart';
 import 'package:aves/widgets/common/icons.dart';
 import 'package:aves/widgets/common/scroll_thumb.dart';
+import 'package:aves/widgets/common/sloppy_scroll_physics.dart';
 import 'package:draggable_scrollbar/draggable_scrollbar.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
@@ -153,7 +154,7 @@ class _CollectionScrollViewState extends State<CollectionScrollView> {
       primary: true,
       // workaround to prevent scrolling the app bar away
       // when there is no content and we use `SliverFillRemaining`
-      physics: collection.isEmpty ? const NeverScrollableScrollPhysics() : null,
+      physics: collection.isEmpty ? const NeverScrollableScrollPhysics() : const SloppyScrollPhysics(parent: AlwaysScrollableScrollPhysics()),
       cacheExtent: widget.cacheExtent,
       slivers: [
         appBar,

lib/widgets/common/sloppy_scroll_physics.dart

@@ -0,0 +1,25 @@
+import 'package:flutter/gestures.dart';
+import 'package:flutter/widgets.dart';
+
+class SloppyScrollPhysics extends ScrollPhysics {
+  const SloppyScrollPhysics({
+    this.touchSlopFactor = 1,
+    ScrollPhysics parent,
+  }) : super(parent: parent);
+
+  // in [0, 1]
+  // 0: most reactive but will not let other recognizers accept gestures
+  // 1: less reactive but gives the most leeway to other recognizers
+  final double touchSlopFactor;
+
+  @override
+  SloppyScrollPhysics applyTo(ScrollPhysics ancestor) {
+    return SloppyScrollPhysics(
+      touchSlopFactor: touchSlopFactor,
+      parent: buildParent(ancestor),
+    );
+  }
+
+  @override
+  double get dragStartDistanceMotionThreshold => kTouchSlop * touchSlopFactor;
+}