Round visual rects when crossing frame boundaries.

third_party/WebKit/Source/core/layout/LayoutView.cpp

Index: third_party/WebKit/Source/core/layout/LayoutView.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutView.cpp b/third_party/WebKit/Source/core/layout/LayoutView.cpp
index b49d539afca42b34114e18eefccf3d55200781ed..de4197ade8f4e2e7ebbb163e8d9001ddbab1963a 100644
--- a/third_party/WebKit/Source/core/layout/LayoutView.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutView.cpp
@@ -529,6 +529,10 @@ bool LayoutView::mapToVisibleRectInAncestorSpace(const LayoutBoxModelObject* anc
             // Adjust for scroll offset of the view.
             rect.moveBy(-viewRectangle.location());
         }
+        // Frames are painted at rounded-int position. Since we cannot efficiently compute the subpixel offset
+        // of painting at this point in a a bottom-up walk, round to the enclosing int rect, which will enclose
+        // the actual visible rect.
+        rect = LayoutRect(enclosingIntRect(rect));
 
         // Adjust for frame border.
         rect.move(obj->contentBoxOffset());