Fix several corner case issues of scrollbar paint invalidation

third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp

Index: third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp b/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
index 6d1bf5e6804f771ffa97a4784cc174588667985c..f6630b9d72c357828223ae72f95d62baced59eb3 100644
--- a/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
@@ -337,6 +337,10 @@ static void forceRecomputePaintInvalidationRectsIncludingNonCompositingDescendan
     // changing the previous position from our paint invalidation container, which is fine as
     // we want a full paint invalidation anyway.
     layoutObject->setPreviousPaintInvalidationRect(LayoutRect());

[chrishtr, 2015/12/03 18:52:36]

Add a new virtual method LayoutObject::clearPreviousPaintInvalidationRects() and
override in LayoutBox?

This would avoid the isBox() stuff, and make the intent of the code here and the
other place clearer.

[Xianzhu, 2015/12/03 22:13:27]

Done.

+    if (layoutObject->isBox()) {
+        if (PaintLayerScrollableArea* scrollableArea = toLayoutBox(layoutObject)->scrollableArea())
+            scrollableArea->clearPreviousPaintInvalidationRects();
+    }
     layoutObject->setShouldDoFullPaintInvalidation();
 
     for (LayoutObject* child = layoutObject->slowFirstChild(); child; child = child->nextSibling()) {