refactor: rely on platform scroll physics and add scroll behavior tests

README.md

@@ -95,10 +95,15 @@ would mean scrolling back to the top just to change what you are looking at.
 Shifts keeps its whole header fixed for the same reason — the Booked / Open /
 History switch has to stay reachable.
 
-Scrolling uses bouncing physics on every platform (`KrowScrollBehavior`). Under
-a pinned header a rubber-band edge tells you that you have reached the end of
-the content rather than the end of the screen; clamping just stops dead. The
-overscroll glow is turned off, since the bounce already says it.
+Scroll physics is left to the platform. Forcing bouncing everywhere looked
+smoother on a long list and broke every short one: `ScrollView` hands any
+vertical list without a controller an `AlwaysScrollableScrollPhysics`, applied
+over whatever the behaviour returns, so the drag is accepted however little
+content there is. Bouncing then let a list that already fits be dragged away
+from the header, leaving a gap down the screen. Android's clamping accepts the
+same drag and pins it to the boundary; iOS bounces, which is its own
+convention. `KrowScrollBehavior` now only adds mouse and trackpad dragging for
+the web and desktop builds.
 
 Two deliberate exceptions:
 

lib/main.dart

@@ -46,12 +46,16 @@ class KrowApp extends StatelessWidget {
 class KrowScrollBehavior extends MaterialScrollBehavior {
   const KrowScrollBehavior();
 
-  /// Bouncing on every platform. The lists here sit under a pinned header, and
-  /// a rubber-band edge shows you have reached the end of the content rather
-  /// than the end of the screen — clamping just stops dead against the header.
-  @override
-  ScrollPhysics getScrollPhysics(BuildContext context) =>
-      const BouncingScrollPhysics(parent: AlwaysScrollableScrollPhysics());
+  // Scroll physics is deliberately left to the platform.
+  //
+  // Forcing BouncingScrollPhysics everywhere looked smoother on a long list
+  // and broke every short one: ScrollView's constructor hands any vertical
+  // list without a controller an AlwaysScrollableScrollPhysics, which is
+  // applied over whatever the behaviour returns and answers
+  // shouldAcceptUserOffset true regardless of content. Bouncing then let
+  // content that already fits be dragged away from the header, leaving a gap
+  // down the screen. Android's clamping accepts the same drag and pins it to
+  // the boundary, so nothing moves; iOS bounces, which is its own convention.
 
   /// Let the web and desktop builds be dragged with a mouse or trackpad, not
   /// only scrolled with a wheel.
@@ -62,12 +66,4 @@ class KrowScrollBehavior extends MaterialScrollBehavior {
     PointerDeviceKind.trackpad,
     PointerDeviceKind.stylus,
   };
-
-  /// The bounce is the end-of-list signal; a glow on top of it is noise.
-  @override
-  Widget buildOverscrollIndicator(
-    BuildContext context,
-    Widget child,
-    ScrollableDetails details,
-  ) => child;
 }

test/scroll_behaviour_test.dart

@@ -0,0 +1,91 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:krow_worker_app/main.dart';
+
+/// A list whose content is shorter than the viewport, so there is nothing to
+/// scroll to.
+Widget shortList() => MaterialApp(
+  scrollBehavior: const KrowScrollBehavior(),
+  home: Scaffold(
+    body: ListView(
+      children: const [SizedBox(height: 100, child: Text('only item'))],
+    ),
+  ),
+);
+
+Widget longList() => MaterialApp(
+  scrollBehavior: const KrowScrollBehavior(),
+  home: Scaffold(
+    body: ListView(
+      children: [
+        for (var i = 0; i < 40; i++) SizedBox(height: 100, child: Text('$i')),
+      ],
+    ),
+  ),
+);
+
+ScrollPosition positionOf(WidgetTester tester) =>
+    tester.state<ScrollableState>(find.byType(Scrollable)).position;
+
+void main() {
+  // TargetPlatformVariant sets and restores the override around each test;
+  // assigning it by hand trips the framework's debug-variable invariant check.
+  final android = TargetPlatformVariant.only(TargetPlatform.android);
+  final ios = TargetPlatformVariant.only(TargetPlatform.iOS);
+
+  group('on Android', () {
+    testWidgets('content that already fits does not move when dragged', (
+      tester,
+    ) async {
+      await tester.pumpWidget(shortList());
+      final position = positionOf(tester);
+
+      // Every vertical list without a controller is handed an
+      // AlwaysScrollableScrollPhysics by ScrollView itself, so the drag is
+      // accepted whatever the content. Clamping is what pins it to the
+      // boundary. Forcing bouncing globally instead let a one-item list be
+      // dragged hundreds of pixels down, leaving a gap under the header.
+      await tester.drag(find.text('only item'), const Offset(0, 300));
+      await tester.pump();
+
+      expect(position.pixels, 0);
+    }, variant: android);
+
+    testWidgets('a list that overflows still scrolls', (tester) async {
+      await tester.pumpWidget(longList());
+      final position = positionOf(tester);
+
+      await tester.drag(find.text('0'), const Offset(0, -200));
+      await tester.pump();
+
+      expect(position.pixels, greaterThan(0));
+    }, variant: android);
+
+    testWidgets('and does not overscroll past the top', (tester) async {
+      await tester.pumpWidget(longList());
+      final position = positionOf(tester);
+
+      await tester.drag(find.byType(Scrollable), const Offset(0, 400));
+      await tester.pump();
+
+      expect(position.pixels, 0);
+    }, variant: android);
+  });
+
+  group('on iOS', () {
+    testWidgets('short content rubber-bands, which is the convention there', (
+      tester,
+    ) async {
+      await tester.pumpWidget(shortList());
+      final position = positionOf(tester);
+
+      await tester.drag(find.text('only item'), const Offset(0, 300));
+      await tester.pump();
+      expect(position.pixels, lessThan(0));
+
+      // And springs back on release, rather than leaving the gap behind.
+      await tester.pumpAndSettle();
+      expect(position.pixels, 0);
+    }, variant: ios);
+  });
+}