Fix several corner case issues of scrollbar paint invalidation

third_party/WebKit/Source/core/paint/PaintInvalidationCapableScrollableArea.h

Index: third_party/WebKit/Source/core/paint/PaintInvalidationCapableScrollableArea.h
diff --git a/third_party/WebKit/Source/core/paint/PaintInvalidationCapableScrollableArea.h b/third_party/WebKit/Source/core/paint/PaintInvalidationCapableScrollableArea.h
index 1120378da9abda5c09dbe7df6eea57d0749e2609..c8fa72b48b122785553a14cdb04630cf63b2c7b2 100644
--- a/third_party/WebKit/Source/core/paint/PaintInvalidationCapableScrollableArea.h
+++ b/third_party/WebKit/Source/core/paint/PaintInvalidationCapableScrollableArea.h
@@ -21,14 +21,23 @@ class PaintInvalidationState;
 // TODO(wangxianzhu): Combine this into PaintLayerScrollableArea when root-layer-scrolls launches.
 class CORE_EXPORT PaintInvalidationCapableScrollableArea : public ScrollableArea {
 public:
+    PaintInvalidationCapableScrollableArea()
+        : m_horizontalScrollbarPreviouslyWasOverlay(false)
+        , m_verticalScrollbarPreviouslyWasOverlay(false) { }
+
     void willRemoveScrollbar(Scrollbar*, ScrollbarOrientation) override;
     void invalidatePaintOfScrollControlsIfNeeded(const PaintInvalidationState&, const LayoutBoxModelObject& paintInvalidationContainer);
 
+    // Should be called when the box is changing paint invalidation container.

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

"Should be called when the previous paint invalidation rects are no longer
valid"

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

Done.

+    void clearPreviousPaintInvalidationRects();
+
 private:
     virtual LayoutBox& boxForScrollControlPaintInvalidation() const = 0;
     virtual LayoutScrollbarPart* scrollCorner() const = 0;
     virtual LayoutScrollbarPart* resizer() const = 0;
 
+    bool m_horizontalScrollbarPreviouslyWasOverlay;

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

Can't you just clear out m_verticalScrollbarPreviousPaintInvalidationRect and
m_horizontalScrollbarPreviousPaintInvalidationRect when changing overlay state
rather than this?

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

No. We need to support the combinations of composited/non-composted and
overlay/non-overlay. For a non-composited non-overlay scrollbar, we need to save
its actual previous paintInvalidationRect for its own paint invalidation, while
check if the scrollbar affects the containing box's padding box size (ignoring
previousPaintInvalidationRect if the scrollbar was previously overlay).

+    bool m_verticalScrollbarPreviouslyWasOverlay;
     LayoutRect m_horizontalScrollbarPreviousPaintInvalidationRect;
     LayoutRect m_verticalScrollbarPreviousPaintInvalidationRect;
     LayoutRect m_scrollCornerPreviousPaintInvalidationRect;