Add comment to RenderColoredBox.paint() (#72526)

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

@@ -7683,6 +7683,10 @@ class _RenderColoredBox extends RenderProxyBoxWithHitTestBehavior {
 
   @override
   void paint(PaintingContext context, Offset offset) {
+    // It's tempting to want to optimize out this `drawRect()` call if the
+    // color is transparent (alpha==0), but doing so would be incorrect. See
+    // https://github.com/flutter/flutter/pull/72526#issuecomment-749185938 for
+    // a good description of why.
     if (size > Size.zero) {
       context.canvas.drawRect(offset & size, Paint()..color = color);
     }