InteractiveViewer with a changing screen size (#60532)

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

@@ -494,11 +494,7 @@ class _InteractiveViewerState extends State<InteractiveViewer> with TickerProvid
 
   // The _boundaryRect is calculated by adding the boundaryMargin to the size of
   // the child.
-  Rect _boundaryRectCached;
   Rect get _boundaryRect {
-    if (_boundaryRectCached != null) {
-      return _boundaryRectCached;
-    }
     assert(_childKey.currentContext != null);
     assert(!widget.boundaryMargin.left.isNaN);
     assert(!widget.boundaryMargin.right.isNaN);
@@ -507,15 +503,15 @@ class _InteractiveViewerState extends State<InteractiveViewer> with TickerProvid
 
     final RenderBox childRenderBox = _childKey.currentContext.findRenderObject() as RenderBox;
     final Size childSize = childRenderBox.size;
-    _boundaryRectCached = widget.boundaryMargin.inflateRect(Offset.zero & childSize);
+    final Rect boundaryRect = widget.boundaryMargin.inflateRect(Offset.zero & childSize);
     // Boundaries that are partially infinite are not allowed because Matrix4's
     // rotation and translation methods don't handle infinites well.
-    assert(_boundaryRectCached.isFinite ||
-        (_boundaryRectCached.left.isInfinite
-        && _boundaryRectCached.top.isInfinite
-        && _boundaryRectCached.right.isInfinite
-        && _boundaryRectCached.bottom.isInfinite), 'boundaryRect must either be infinite in all directions or finite in all directions.');
-    return _boundaryRectCached;
+    assert(boundaryRect.isFinite ||
+        (boundaryRect.left.isInfinite
+        && boundaryRect.top.isInfinite
+        && boundaryRect.right.isInfinite
+        && boundaryRect.bottom.isInfinite), 'boundaryRect must either be infinite in all directions or finite in all directions.');
+    return boundaryRect;
   }
 
   // The Rect representing the child's parent.
@@ -917,10 +913,6 @@ class _InteractiveViewerState extends State<InteractiveViewer> with TickerProvid
   @override
   void didUpdateWidget(InteractiveViewer oldWidget) {
     super.didUpdateWidget(oldWidget);
-    if (widget.child != oldWidget.child || widget.boundaryMargin != oldWidget.boundaryMargin) {
-      _boundaryRectCached = null;
-    }
-
     // Handle all cases of needing to dispose and initialize
     // transformationControllers.
     if (oldWidget.transformationController == null) {

packages/flutter/test/widgets/interactive_viewer_test.dart

@@ -450,6 +450,56 @@ void main() {
 
       expect(transformationController.value.getMaxScaleOnAxis(), equals(1.0));
     });
+
+    testWidgets('viewport changes size', (WidgetTester tester) async {
+      final TransformationController transformationController = TransformationController();
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: Center(
+              child: InteractiveViewer(
+                transformationController: transformationController,
+                child: Container(),
+              ),
+            ),
+          ),
+        ),
+      );
+
+      expect(transformationController.value, equals(Matrix4.identity()));
+
+      // Attempting to drag to pan doesn't work because the child fits inside
+      // the viewport and has a tight boundary.
+      final Offset childOffset = tester.getTopLeft(find.byType(Container));
+      final Offset childInterior = Offset(
+        childOffset.dx + 20.0,
+        childOffset.dy + 20.0,
+      );
+      TestGesture gesture = await tester.startGesture(childInterior);
+      addTearDown(gesture.removePointer);
+      await tester.pump();
+      await gesture.moveTo(childOffset);
+      await tester.pump();
+      await gesture.up();
+      await tester.pumpAndSettle();
+      expect(transformationController.value, equals(Matrix4.identity()));
+
+      // Shrink the size of the screen.
+      tester.binding.window.physicalSizeTestValue = const Size(100.0, 100.0);
+      addTearDown(tester.binding.window.clearPhysicalSizeTestValue);
+      await tester.pump();
+
+      // Attempting to drag to pan still doesn't work, because the image has
+      // resized itself to fit the new screen size, and InteractiveViewer has
+      // updated its measurements to take that into consideration.
+      gesture = await tester.startGesture(childInterior);
+      await tester.pump();
+      await gesture.moveTo(childOffset);
+      await tester.pump();
+      await gesture.up();
+      await tester.pumpAndSettle();
+      expect(transformationController.value, equals(Matrix4.identity()));
+    });
   });
 
   group('getNearestPointOnLine', () {