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lib/src/scroll_forwarder.dart

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+import "package:flutter/foundation.dart";
+import "package:flutter/gestures.dart";
+import "package:flutter/material.dart";
+
+/// Forwards unclaimed pointer scroll interactions within [child]'s bounds to
+/// the position resolved by [positionProvider].
+///
+/// A scrollable only reacts to mouse wheel input while the pointer hovers
+/// directly over it, and only reacts to drags that begin on it. Wrapping a
+/// layout with this widget lets surrounding areas (for example the empty
+/// margins beside a centered list) also drive the target scrollable, via
+/// wheel/trackpad scroll signals or touch drags. A nested scrollable always
+/// claims the interaction first — scroll signals through
+/// [PointerSignalResolver] and drags through the gesture arena — so the target
+/// only scrolls when the pointer is not interacting with another scrollable.
+class ScrollForwarder extends StatefulWidget {
+  const ScrollForwarder({
+    required this.positionProvider,
+    required this.child,
+    super.key,
+  });
+
+  /// Resolves the scroll position to receive unclaimed scroll interactions.
+  ///
+  /// Invoked for every signal or gesture so the current position is always
+  /// used. Return `null` to ignore the interaction.
+  final ScrollPosition? Function() positionProvider;
+
+  final Widget child;
+
+  @override
+  State<ScrollForwarder> createState() => _ScrollForwarderState();
+}
+
+class _ScrollForwarderState extends State<ScrollForwarder> {
+  Drag? _drag;
+  Set<PointerDeviceKind> _dragDevices = const <PointerDeviceKind>{};
+  Map<Type, GestureRecognizerFactory> _gestureRecognizers =
+      const <Type, GestureRecognizerFactory>{};
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+    final Set<PointerDeviceKind> devices = ScrollConfiguration.of(
+      context,
+    ).dragDevices;
+    if (setEquals(devices, _dragDevices)) return;
+
+    _dragDevices = devices;
+    _gestureRecognizers = <Type, GestureRecognizerFactory>{
+      VerticalDragGestureRecognizer:
+          GestureRecognizerFactoryWithHandlers<VerticalDragGestureRecognizer>(
+            () => VerticalDragGestureRecognizer(supportedDevices: devices),
+            (VerticalDragGestureRecognizer instance) {
+              instance
+                ..onStart = _onDragStart
+                ..onUpdate = _onDragUpdate
+                ..onEnd = _onDragEnd
+                ..onCancel = _onDragCancel;
+            },
+          ),
+    };
+  }
+
+  @override
+  void dispose() {
+    _drag?.cancel();
+    super.dispose();
+  }
+
+  void _onPointerSignal(PointerSignalEvent event) {
+    if (event is! PointerScrollEvent || event.scrollDelta.dy == 0.0) return;
+
+    final ScrollPosition? position = widget.positionProvider();
+    if (position == null || !position.hasContentDimensions) return;
+
+    GestureBinding.instance.pointerSignalResolver.register(event, (_) {
+      position.pointerScroll(event.scrollDelta.dy);
+      event.respond(allowPlatformDefault: false);
+    });
+  }
+
+  void _onDragStart(DragStartDetails details) {
+    final ScrollPosition? position = widget.positionProvider();
+    if (position == null) return;
+    _drag = position.drag(details, _disposeDrag);
+  }
+
+  void _disposeDrag() {
+    _drag = null;
+  }
+
+  void _onDragUpdate(DragUpdateDetails details) {
+    _drag?.update(details);
+  }
+
+  void _onDragEnd(DragEndDetails details) {
+    _drag?.end(details);
+  }
+
+  void _onDragCancel() {
+    _drag?.cancel();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Listener(
+      behavior: HitTestBehavior.translucent,
+      onPointerSignal: _onPointerSignal,
+      child: RawGestureDetector(
+        behavior: HitTestBehavior.translucent,
+        gestures: _gestureRecognizers,
+        child: widget.child,
+      ),
+    );
+  }
+}