Draw debug rects when flex overflows

packages/flutter/lib/rendering/flex.dart

@@ -81,7 +81,8 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
     }
   }
 
-  bool _overflowOccurredDuringLayout = false;
+  // Set during layout if overflow occurred on the main axis
+  double _overflow;
 
   void setupParentData(RenderBox child) {
     if (child.parentData is! FlexBoxParentData)
@@ -314,6 +315,8 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
       }
       child = child.parentData.nextSibling;
     }
+    _overflow = math.max(0.0, -freeSpace);
+    freeSpace = math.max(0.0, freeSpace);
 
     // Steps 4-5. Distribute remaining space to flexible children.
     double spacePerFlex = totalFlex > 0 ? (freeSpace / totalFlex) : 0.0;
@@ -351,7 +354,7 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
       child = child.parentData.nextSibling;
     }
 
-    // Section 8.2: Axis Alignment using the justify-content property
+    // Section 8.2: Main Axis Alignment using the justify-content property
     double remainingSpace = math.max(0.0, freeSpace - usedSpace);
     double leadingSpace;
     double betweenSpace;
@@ -378,20 +381,16 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
         break;
     }
 
-    Size desiredSize;
     switch (_direction) {
       case FlexDirection.horizontal:
-        desiredSize = new Size(mainSize, crossSize);
-        size = constraints.constrain(desiredSize);
+        size = constraints.constrain(new Size(mainSize, crossSize));
         crossSize = size.height;
         break;
       case FlexDirection.vertical:
-        desiredSize = new Size(crossSize, mainSize);
         size = constraints.constrain(new Size(crossSize, mainSize));
         crossSize = size.width;
         break;
     }
-    _overflowOccurredDuringLayout = desiredSize != size;
 
     // Position elements
     double childMainPosition = leadingSpace;
@@ -439,7 +438,25 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
   }
 
   void paint(PaintingCanvas canvas, Offset offset) {
-    if (_overflowOccurredDuringLayout) {
+    if (_overflow > 0) {
+      assert(() {
+        // Draw a red rectangle over the overflow area in debug mode
+        // You should be using a Clip if you want to clip your children
+        Paint paint = new Paint()..color = const Color(0x7FFF0000);
+        Rect overflowRect;
+        switch(direction) {
+          case FlexDirection.horizontal:
+            overflowRect = offset + new Offset(size.width, 0.0) &
+                           new Size(_overflow, size.height);
+            break;
+          case FlexDirection.vertical:
+            overflowRect = offset + new Offset(0.0, size.height) &
+                           new Size(size.width, _overflow);
+            break;
+        }
+        canvas.drawRect(overflowRect, paint);
+        return true;
+      });
       canvas.save();
       canvas.clipRect(offset & size);
       defaultPaint(canvas, offset);