Don't adjust for scrolling of the ancestor, rather than un-adjusting for it later.

third_party/WebKit/Source/core/layout/LayoutObject.h

Index: third_party/WebKit/Source/core/layout/LayoutObject.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutObject.h b/third_party/WebKit/Source/core/layout/LayoutObject.h
index 7a88a69240315972c9088971c8606a36422c663f..06f2c77b20d87b9d044c6e7d3d0bf69e63503c16 100644
--- a/third_party/WebKit/Source/core/layout/LayoutObject.h
+++ b/third_party/WebKit/Source/core/layout/LayoutObject.h
@@ -1142,9 +1142,9 @@ public:
 
     // Given a rect in the object's coordinate space, mutates the rect into one representing the size of its visual painted
     // output as if |ancestor| was the root of the page: the rect is modified by any intervening clips, transforms
-    // and scrolls between |this| and |ancestor|, but not any above |ancestor|.
+    // and scrolls between |this| and |ancestor| (not inclusive of |ancestor|), but not any above |ancestor|.
     // The output is in the physical, painted coordinate pixel space of |ancestor|.
-    // Overflow clipping is *not* applied for |ancestor| itself if |ancestor| scrolls overflow.
+    // Overflow clipping and scrolling is *not* applied for |ancestor| itself if |ancestor| scrolls overflow.
     // The output rect is suitable for purposes such as paint invalidation.
     //
     // If visualRectFlags has the EdgeInclusive bit set, clipping operations will use