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

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;
 }