2DScrollView - Fix drag when one axis does not have enough content (#145566)

Fixes https://github.com/flutter/flutter/issues/144982
For reference, may have to do with #138442 when we reworked the gesture handling. The adjustments to the comments here were certainly from #138442 not updating them. Confused myself for a minute or two. 🙃

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

@@ -1982,6 +1982,7 @@ class TwoDimensionalScrollableState extends State<TwoDimensionalScrollable> {
         viewportBuilder: (BuildContext context, ViewportOffset verticalOffset) {
           return _HorizontalInnerDimension(
             key: _horizontalInnerScrollableKey,
+            verticalOuterKey: _verticalOuterScrollableKey,
             axisDirection: widget.horizontalDetails.direction,
             controller: widget.horizontalDetails.controller
               ?? _horizontalFallbackController!,
@@ -2250,9 +2251,9 @@ class _VerticalOuterDimensionState extends ScrollableState {
         if (value) {
           // Replaces the typical vertical/horizontal drag gesture recognizers
           // with a pan gesture recognizer to allow bidirectional scrolling.
-          // Based on the diagonalDragBehavior, valid horizontal deltas are
-          // applied to this scrollable, while vertical deltas are routed to
-          // the vertical scrollable.
+          // Based on the diagonalDragBehavior, valid vertical deltas are
+          // applied to this scrollable, while horizontal deltas are routed to
+          // the horizontal scrollable.
           _gestureRecognizers = <Type, GestureRecognizerFactory>{
             PanGestureRecognizer: GestureRecognizerFactoryWithHandlers<PanGestureRecognizer>(
               () => PanGestureRecognizer(supportedDevices: _configuration.dragDevices),
@@ -2303,6 +2304,7 @@ class _VerticalOuterDimensionState extends ScrollableState {
 class _HorizontalInnerDimension extends Scrollable {
   const _HorizontalInnerDimension({
     super.key,
+    required this.verticalOuterKey,
     required super.viewportBuilder,
     required super.axisDirection,
     super.controller,
@@ -2315,6 +2317,7 @@ class _HorizontalInnerDimension extends Scrollable {
     this.diagonalDragBehavior = DiagonalDragBehavior.none,
   }) : assert(axisDirection == AxisDirection.left || axisDirection == AxisDirection.right);
 
+  final GlobalKey<ScrollableState> verticalOuterKey;
   final DiagonalDragBehavior diagonalDragBehavior;
 
   @override
@@ -2324,6 +2327,7 @@ class _HorizontalInnerDimension extends Scrollable {
 class _HorizontalInnerDimensionState extends ScrollableState {
   late ScrollableState verticalScrollable;
 
+  GlobalKey<ScrollableState> get verticalOuterKey => (widget as _HorizontalInnerDimension).verticalOuterKey;
   DiagonalDragBehavior get diagonalDragBehavior => (widget as _HorizontalInnerDimension).diagonalDragBehavior;
 
   @override
@@ -2379,9 +2383,12 @@ class _HorizontalInnerDimensionState extends ScrollableState {
       case DiagonalDragBehavior.free:
         if (value) {
           // If a type of diagonal scrolling is enabled, a panning gesture
-          // recognizer will be created for the _InnerDimension. So in this
-          // case, the _OuterDimension does not require a gesture recognizer.
+          // recognizer will be created for the _VerticalOuterDimension. So in
+          // this case, the _HorizontalInnerDimension does not require a gesture
+          // recognizer, meanwhile we should ensure the outer dimension has
+          // updated in case it did not have enough content to enable dragging.
           _gestureRecognizers = const <Type, GestureRecognizerFactory>{};
+          verticalOuterKey.currentState!.setCanDrag(value);
           // Cancel the active hold/drag (if any) because the gesture recognizers
           // will soon be disposed by our RawGestureDetector, and we won't be
           // receiving pointer up events to cancel the hold/drag.