Update viewport dimensions when scrollDirection changes (#61973)

packages/flutter/lib/src/rendering/viewport.dart

@@ -1321,16 +1321,6 @@ class RenderViewport extends RenderViewportBase<SliverPhysicalContainerParentDat
       return true;
     }());
     size = constraints.biggest;
-    // We ignore the return value of applyViewportDimension below because we are
-    // going to go through performLayout next regardless.
-    switch (axis) {
-      case Axis.vertical:
-        offset.applyViewportDimension(size.height);
-        break;
-      case Axis.horizontal:
-        offset.applyViewportDimension(size.width);
-        break;
-    }
   }
 
   static const int _maxLayoutCycles = 10;
@@ -1342,6 +1332,17 @@ class RenderViewport extends RenderViewportBase<SliverPhysicalContainerParentDat
 
   @override
   void performLayout() {
+    // Ignore the return value of applyViewportDimension because we are
+    // doing a layout regardless.
+    switch (axis) {
+      case Axis.vertical:
+        offset.applyViewportDimension(size.height);
+        break;
+      case Axis.horizontal:
+        offset.applyViewportDimension(size.width);
+        break;
+    }
+
     if (center == null) {
       assert(firstChild == null);
       _minScrollExtent = 0.0;

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

@@ -395,6 +395,9 @@ class _PagePosition extends ScrollPositionWithSingleContext implements PageMetri
   @override
   bool applyViewportDimension(double viewportDimension) {
     final double oldViewportDimensions = this.viewportDimension;
+    if (viewportDimension == oldViewportDimensions) {
+      return true;
+    }
     final bool result = super.applyViewportDimension(viewportDimension);
     final double oldPixels = pixels;
     final double page = (oldPixels == null || oldViewportDimensions == 0.0) ? _pageToUseOnStartup : getPageFromPixels(oldPixels, oldViewportDimensions);

packages/flutter/test/widgets/list_view_test.dart

@@ -537,4 +537,42 @@ void main() {
     ));
     handle.dispose();
   });
+
+  testWidgets('Updates viewport dimensions when scroll direction changes', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/43380.
+    final ScrollController controller = ScrollController();
+
+    Widget buildListView({@required Axis scrollDirection}) {
+      assert(scrollDirection != null);
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: Center(
+          child: Container(
+            height: 200.0,
+            width: 100.0,
+            child: ListView(
+              controller: controller,
+              scrollDirection: scrollDirection,
+              itemExtent: 50.0,
+              children: <Widget>[
+                Container(
+                  height: 50.0,
+                  width: 50.0,
+                ),
+              ],
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildListView(scrollDirection: Axis.horizontal));
+    expect(controller.position.viewportDimension, 100.0);
+
+    await tester.pumpWidget(buildListView(scrollDirection: Axis.vertical));
+    expect(controller.position.viewportDimension, 200.0);
+
+    await tester.pumpWidget(buildListView(scrollDirection: Axis.horizontal));
+    expect(controller.position.viewportDimension, 100.0);
+  });
 }