Fix paint offset in reverse for 2D (#128724)

Fixes https://github.com/flutter/flutter/issues/128723

The paint offset was incorrectly being computed when one or both axis was in the reverse direction. Instead of using the paint extent, the child's size should be used.

packages/flutter/lib/src/widgets/two_dimensional_viewport.dart

@@ -1149,7 +1149,6 @@ abstract class RenderTwoDimensionalViewport extends RenderBox implements RenderA
     childParentData.paintOffset = computeAbsolutePaintOffsetFor(
       child,
       layoutOffset: childParentData.layoutOffset!,
-      paintExtent: childParentData._paintExtent!,
     );
     // If the child is partially visible, or not visible at all, there is
     // visual overflow.
@@ -1243,14 +1242,15 @@ abstract class RenderTwoDimensionalViewport extends RenderBox implements RenderA
   Offset computeAbsolutePaintOffsetFor(
     RenderBox child, {
     required Offset layoutOffset,
-    required Size paintExtent,
   }) {
-    assert(hasSize); // this is only usable once we have a size
+    // This is only usable once we have sizes.
+    assert(hasSize);
+    assert(child.hasSize);
     final double xOffset;
     final double yOffset;
     switch (verticalAxisDirection) {
       case AxisDirection.up:
-        yOffset = viewportDimension.height - (layoutOffset.dy + paintExtent.height);
+        yOffset = viewportDimension.height - (layoutOffset.dy + child.size.height);
       case AxisDirection.down:
         yOffset = layoutOffset.dy;
       case AxisDirection.right:
@@ -1261,7 +1261,7 @@ abstract class RenderTwoDimensionalViewport extends RenderBox implements RenderA
       case AxisDirection.right:
         xOffset = layoutOffset.dx;
       case AxisDirection.left:
-        xOffset = viewportDimension.width - (layoutOffset.dx + paintExtent.width);
+        xOffset = viewportDimension.width - (layoutOffset.dx + child.size.width);
       case AxisDirection.up:
       case AxisDirection.down:
         throw Exception('This should not happen');

packages/flutter/test/widgets/two_dimensional_viewport_test.dart

@@ -1189,7 +1189,8 @@ void main() {
     }, variant: TargetPlatformVariant.all());
 
     testWidgets('sets up parent data', (WidgetTester tester) async {
-      // Also tests computeChildPaintOffset & computeChildPaintExtent
+      // Also tests computeAbsolutePaintOffsetFor & computeChildPaintExtent
+      // Regression test for https://github.com/flutter/flutter/issues/128723
       final Map<ChildVicinity, UniqueKey> childKeys = <ChildVicinity, UniqueKey>{};
       final TwoDimensionalChildBuilderDelegate delegate = TwoDimensionalChildBuilderDelegate(
         maxXIndex: 5,
@@ -1250,7 +1251,7 @@ void main() {
       childParentData = parentDataOf(viewport.lastChild!);
       expect(childParentData.vicinity, const ChildVicinity(xIndex: 5, yIndex: 5));
       expect(childParentData.isVisible, isFalse);
-      expect(childParentData.paintOffset, const Offset(1000.0, -400.0));
+      expect(childParentData.paintOffset, const Offset(1000.0, -600.0));
       expect(childParentData.layoutOffset, const Offset(1000.0, 1000.0));
 
       // - horizontal reverse
@@ -1271,7 +1272,7 @@ void main() {
       childParentData = parentDataOf(viewport.lastChild!);
       expect(childParentData.vicinity, const ChildVicinity(xIndex: 5, yIndex: 5));
       expect(childParentData.isVisible, isFalse);
-      expect(childParentData.paintOffset, const Offset(-200.0, 1000.0));
+      expect(childParentData.paintOffset, const Offset(-400.0, 1000.0));
       expect(childParentData.layoutOffset, const Offset(1000.0, 1000.0));
 
       // - both reverse
@@ -1293,7 +1294,7 @@ void main() {
       childParentData = parentDataOf(viewport.lastChild!);
       expect(childParentData.vicinity, const ChildVicinity(xIndex: 5, yIndex: 5));
       expect(childParentData.isVisible, isFalse);
-      expect(childParentData.paintOffset, const Offset(-200.0, -400.0));
+      expect(childParentData.paintOffset, const Offset(-400.0, -600.0));
       expect(childParentData.layoutOffset, const Offset(1000.0, 1000.0));
 
       // Change the scroll positions to test partially visible.