fix(cursor): align the remote cursor with scrollbar offsets

flutter/lib/desktop/pages/remote_page.dart

@@ -1401,22 +1401,20 @@ class CursorPaint extends StatelessWidget {
     final imageOffset = _softwareImageOffset(c);
     double cx = imageOffset?.dx ?? c.x;
     double cy = imageOffset?.dy ?? c.y;
-    if (c.viewStyle.style == kRemoteViewStyleOriginal &&
-        c.scrollStyle == ScrollStyle.scrollbar) {
+    if (c.imageOverflow.isTrue && c.scrollStyle == ScrollStyle.scrollbar) {
       final rect = c.parent.target!.ffiModel.rect;
       if (rect == null) {
         // unreachable!
         debugPrint('unreachable! The displays rect is null.');
         return Container();
       }
-      if (cx < 0) {
-        final imageWidth = rect.width * c.scale;
-        cx = -imageWidth * c.scrollX;
-      }
-      if (cy < 0) {
-        final imageHeight = rect.height * c.scale;
-        cy = -imageHeight * c.scrollY;
-      }
+      // Pan offsets can be stale after leaving the image; scrollbars do not use them.
+      final imageWidth = rect.width * c.scale;
+      final imageHeight = rect.height * c.scale;
+      cx = (c.size.width > imageWidth ? (c.size.width - imageWidth) / 2 : 0) -
+          imageWidth * c.scrollX;
+      cy = (c.size.height > imageHeight ? (c.size.height - imageHeight) / 2 : 0) -
+          imageHeight * c.scrollY;
     }
 
     final image = m.image ?? preDefaultCursor.image;

flutter/test/cursor_paint_scale_test.dart

@@ -45,6 +45,8 @@ class _Peer extends Fake implements FfiModel {
   final pi = PeerInfo();
   @override
   bool get isPeerLinux => false;
+  @override
+  Rect get rect => Offset.zero & _viewport;
 }
 
 class _FFI extends Fake implements FFI {
@@ -82,11 +84,26 @@ class _CanvasModel extends ChangeNotifier implements CanvasModel {
   final double scale;
   @override
   ScrollStyle get scrollStyle => ScrollStyle.scrollauto;
+  @override
+  Size get size => _viewport;
 
   @override
   dynamic noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 }
 
+class _ScrollbarCanvasModel extends _CanvasModel {
+  _ScrollbarCanvasModel(String style) : super(style, 2, true) {
+    imageOverflow.value = true;
+  }
+
+  @override
+  ScrollStyle get scrollStyle => ScrollStyle.scrollbar;
+  @override
+  double get scrollX => 0.1;
+  @override
+  double get scrollY => 0.2;
+}
+
 class _Canvas extends Fake implements Canvas {
   double factor = 1;
   Offset? position;
@@ -161,4 +178,31 @@ void main() {
       expect(position.dy, closeTo(target.dy, 1e-9));
     });
   }
+  for (final style in [kRemoteViewStyleOriginal, kRemoteViewStyleCustom]) {
+    testWidgets('$style painted cursor follows scrollbar position',
+        (tester) async {
+      final image = (await tester.runAsync(
+          () => createTestImage(width: 48, height: 64)))!;
+      addTearDown(image.dispose);
+      await tester.pumpWidget(MediaQuery(
+        data: const MediaQueryData(devicePixelRatio: 2),
+        child: MultiProvider(
+          providers: [
+            ChangeNotifierProvider<CursorModel>(
+                create: (_) => _CursorModel(image)),
+            ChangeNotifierProvider<CanvasModel>(
+                create: (_) => _ScrollbarCanvasModel(style)),
+          ],
+          child: CursorPaint(id: 'cursor-test', zoomCursor: true.obs),
+        ),
+      ));
+      final painter = tester
+          .widget<CustomPaint>(find.byType(CustomPaint))
+          .painter! as ImagePainter;
+      final target = _remotePosition * 2 -
+          Offset(_viewport.width * 2 * 0.1, _viewport.height * 2 * 0.2);
+      expect(
+          (Offset(painter.x, painter.y) + _hotspot) * painter.scale, target);
+    });
+  }
 }