Avoid NaN for shrinkwrapping viewports (#46265)

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

@@ -505,7 +505,16 @@ abstract class RenderViewportBase<ParentDataClass extends ContainerParentDataMix
   @override
   Rect describeApproximatePaintClip(RenderSliver child) {
     final Rect viewportClip = Offset.zero & size;
-    if (child.constraints.overlap == 0) {
+    // The child's viewportMainAxisExtent can be infinite when a
+    // RenderShrinkWrappingViewport is given infinite constraints, such as when
+    // it is the child of a Row or Column (depending on orientation).
+    //
+    // For example, a shrink wrapping render sliver may have infinite
+    // constraints along the viewport's main axis but may also have bouncing
+    // scroll physics, which will allow for some scrolling effect to occur.
+    // We should just use the viewportClip - the start of the overlap is at
+    // double.infinity and so it is effectively meaningless.
+    if (child.constraints.overlap == 0 || !child.constraints.viewportMainAxisExtent.isFinite) {
       return viewportClip;
     }
 
@@ -1756,6 +1765,11 @@ class RenderShrinkWrappingViewport extends RenderViewportBase<SliverLogicalConta
   }
 
   double _attemptLayout(double mainAxisExtent, double crossAxisExtent, double correctedOffset) {
+    // We can't assert mainAxisExtent is finite, because it could be infinite if
+    // it is within a column or row for example. In such a case, there's not
+    // even any scrolling to do, although some scroll physics (i.e.
+    // BouncingScrollPhysics) could still temporarily scroll the content in a
+    // simulation.
     assert(!mainAxisExtent.isNaN);
     assert(mainAxisExtent >= 0.0);
     assert(crossAxisExtent.isFinite);

packages/flutter/test/rendering/viewport_caching_test.dart

@@ -124,4 +124,47 @@ void main() {
     layout(renderViewport);
     expect(renderViewport.describeSemanticsClip(null), rectExpandedOnAxis(height * 2.5));
   });
+
+  test('RenderShrinkWrappingViewport describeApproximatePaintClip with infinite viewportMainAxisExtent returns finite rect', () {
+    final RenderSliver child = CustomConstraintsRenderSliver(const SliverConstraints(
+      axisDirection: AxisDirection.down,
+      cacheOrigin: 0.0,
+      crossAxisDirection: AxisDirection.left,
+      crossAxisExtent: 400.0,
+      growthDirection: GrowthDirection.forward,
+      overlap: 1.0,                                // must not equal 0 for this test
+      precedingScrollExtent: 0.0,
+      remainingPaintExtent: double.infinity,
+      remainingCacheExtent: 0.0,
+      scrollOffset: 0.0,
+      userScrollDirection: ScrollDirection.idle,
+      viewportMainAxisExtent: double.infinity,     // must == infinity
+    ));
+
+    final RenderShrinkWrappingViewport viewport = RenderShrinkWrappingViewport(
+      crossAxisDirection: AxisDirection.left,
+      offset: ViewportOffset.zero(),
+      children: <RenderSliver>[ child ],
+    );
+
+    layout(viewport);
+    expect(
+      viewport.describeApproximatePaintClip(child),
+      const Rect.fromLTRB(0.0, 0.0, 800.0, 600.0),
+    );
+  });
+}
+
+class CustomConstraintsRenderSliver extends RenderSliver {
+  CustomConstraintsRenderSliver(this.constraints);
+
+  @override
+  SliverGeometry get geometry => const SliverGeometry();
+
+  @override
+  final SliverConstraints constraints;
+
+  @override
+  void performLayout() {}
+
 }

packages/flutter/test/rendering/viewport_test.dart

@@ -1144,4 +1144,39 @@ void main() {
       '   its intrinsic dimensions.\n',
     );
   });
+
+  testWidgets('Handles infinite constraints when TargetPlatform is iOS', (WidgetTester tester) async {
+    // regression test for https://github.com/flutter/flutter/issues/45866
+    final TargetPlatform oldTargetPlatform = debugDefaultTargetPlatformOverride;
+    debugDefaultTargetPlatformOverride = TargetPlatform.iOS;
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Column(
+          crossAxisAlignment: CrossAxisAlignment.start,
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: <Widget>[
+            GridView(
+              shrinkWrap: true,
+              gridDelegate: const SliverGridDelegateWithFixedCrossAxisCount(
+                  crossAxisCount: 3,
+                  childAspectRatio: 3,
+                  mainAxisSpacing: 3,
+                  crossAxisSpacing: 3),
+              children: const <Widget>[
+                Text('a'),
+                Text('b'),
+                Text('c'),
+              ],
+            ),
+          ],
+        ),
+      ),
+    );
+
+    expect(find.text('b'), findsOneWidget);
+    await tester.drag(find.text('b'), const Offset(0, 200));
+    await tester.pumpAndSettle();
+    debugDefaultTargetPlatformOverride = oldTargetPlatform;
+  });
 }