Fix FittedBox BoxFit.scaleDown sizing (#63668)

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

@@ -2352,6 +2352,15 @@ class RenderFittedBox extends RenderProxyBox {
     markNeedsPaint();
   }
 
+  bool _fitAffectsLayout(BoxFit fit) {
+    switch (fit) {
+      case BoxFit.scaleDown:
+        return true;
+      default:
+        return false;
+    }
+  }
+
   /// How to inscribe the child into the space allocated during layout.
   BoxFit get fit => _fit;
   BoxFit _fit;
@@ -2359,9 +2368,14 @@ class RenderFittedBox extends RenderProxyBox {
     assert(value != null);
     if (_fit == value)
       return;
+    final BoxFit lastFit = _fit;
     _fit = value;
-    _clearPaintData();
-    markNeedsPaint();
+    if (_fitAffectsLayout(lastFit) || _fitAffectsLayout(value)) {
+      markNeedsLayout();
+    } else {
+      _clearPaintData();
+      markNeedsPaint();
+    }
   }
 
   /// How to align the child within its parent's bounds.
@@ -2403,7 +2417,16 @@ class RenderFittedBox extends RenderProxyBox {
   void performLayout() {
     if (child != null) {
       child.layout(const BoxConstraints(), parentUsesSize: true);
-      size = constraints.constrainSizeAndAttemptToPreserveAspectRatio(child.size);
+      switch (fit) {
+        case BoxFit.scaleDown:
+          final BoxConstraints sizeConstraints = constraints.loosen();
+          final Size unconstrainedSize = sizeConstraints.constrainSizeAndAttemptToPreserveAspectRatio(child.size);
+          size = constraints.constrain(unconstrainedSize);
+          break;
+        default:
+          size = constraints.constrainSizeAndAttemptToPreserveAspectRatio(child.size);
+          break;
+      }
       _clearPaintData();
     } else {
       size = constraints.smallest;

packages/flutter/test/widgets/fitted_box_test.dart

@@ -486,6 +486,111 @@ void main() {
     await tester.pumpWidget(FittedBox(fit: BoxFit.none, child: Container(), clipBehavior: Clip.antiAlias));
     expect(renderObject.clipBehavior, equals(Clip.antiAlias));
   });
+
+  testWidgets('BoxFit.scaleDown matches size of child', (WidgetTester tester) async {
+    final Key outside = UniqueKey();
+    final Key inside = UniqueKey();
+
+    // Does not scale up when child is smaller than constraints
+
+    await tester.pumpWidget(
+      Center(
+        child: Container(
+          width: 200.0,
+          child: FittedBox(
+            key: outside,
+            fit: BoxFit.scaleDown,
+            child: Container(
+              key: inside,
+              width: 100.0,
+              height: 50.0,
+            ),
+          ),
+        ),
+      ),
+    );
+
+    final RenderBox outsideBox = tester.firstRenderObject(find.byKey(outside));
+    final RenderBox insideBox = tester.firstRenderObject(find.byKey(inside));
+
+    expect(outsideBox.size.width, 200.0);
+    expect(outsideBox.size.height, 50.0);
+
+    Offset outsidePoint = outsideBox.localToGlobal(Offset.zero);
+    Offset insidePoint = insideBox.localToGlobal(Offset.zero);
+    expect(insidePoint - outsidePoint, equals(const Offset(50.0, 0.0)));
+
+    // Scales down when child is bigger than constraints
+
+    await tester.pumpWidget(
+      Center(
+        child: Container(
+          width: 200.0,
+          child: FittedBox(
+            key: outside,
+            fit: BoxFit.scaleDown,
+            child: Container(
+              key: inside,
+              width: 400.0,
+              height: 200.0,
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(outsideBox.size.width, 200.0);
+    expect(outsideBox.size.height, 100.0);
+
+    outsidePoint = outsideBox.localToGlobal(Offset.zero);
+    insidePoint = insideBox.localToGlobal(Offset.zero);
+
+    expect(insidePoint - outsidePoint, equals(Offset.zero));
+  });
+
+  testWidgets('Switching to and from BoxFit.scaleDown causes relayout', (WidgetTester tester) async {
+    final Key outside = UniqueKey();
+
+    final Widget scaleDownWidget = Center(
+      child: Container(
+        width: 200.0,
+        child: FittedBox(
+          key: outside,
+          fit: BoxFit.scaleDown,
+          child: Container(
+            width: 100.0,
+            height: 50.0,
+          ),
+        ),
+      ),
+    );
+
+    final Widget coverWidget = Center(
+      child: Container(
+        width: 200.0,
+        child: FittedBox(
+          key: outside,
+          child: Container(
+            width: 100.0,
+            height: 50.0,
+          ),
+        ),
+      ),
+    );
+
+    await tester.pumpWidget(scaleDownWidget);
+
+    final RenderBox outsideBox = tester.firstRenderObject(find.byKey(outside));
+    expect(outsideBox.size.height, 50.0);
+
+    await tester.pumpWidget(coverWidget);
+
+    expect(outsideBox.size.height, 100.0);
+
+    await tester.pumpWidget(scaleDownWidget);
+
+    expect(outsideBox.size.height, 50.0);
+  });
 }
 
 List<Type> getLayers() {