Calculate paint invalidation rect for scrollbars

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
new file mode 100644
index 0000000000000000000000000000000000000000..4fbe1ab14a50aeca0991678d70f35dbb41ab03e4
--- /dev/null
+++ b/third_party/WebKit/Source/core/paint/PaintInvalidationCapableScrollableArea.h
@@ -0,0 +1,40 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef PaintInvalidationCapableScrollableArea_h
+#define PaintInvalidationCapableScrollableArea_h
+
+#include "core/CoreExport.h"
+#include "platform/geometry/LayoutRect.h"
+#include "platform/scroll/ScrollableArea.h"
+
+namespace blink {
+
+class LayoutBox;
+class LayoutBoxModelObject;
+class LayoutScrollbarPart;
+class PaintInvalidationContainer;
+class PaintInvalidationState;
+
+// Base class of FrameView and PaintLayerScrollableArea to share paint invalidation code.
+// TODO(wangxianzhu): Combine this into PaintLayerScrollableArea when FrameView no longer scrolls.
+class CORE_EXPORT PaintInvalidationCapableScrollableArea : public ScrollableArea {
+public:
+    // The LayoutBox parameter is the LayoutView of a FrameView if the scrollbars belong to the FrameView,
+    // or the box() of a PaintLayerScrollbarArea if the scrollbars belong to the PaintLayerScrollableArea.
+    void willRemoveScrollbar(LayoutBox&, Scrollbar*, ScrollbarOrientation);

[chrishtr, 2015/11/17 00:44:32]

I don't think the first argument is necessary. Both subclasses can determine the
appropriate
LayoutObject, which is always the LayoutView in the case of FrameView scrolling,
or the associated
LayoutBox with the scrollable area.

[Xianzhu, 2015/11/17 00:56:05]

The two cases share common code in this class which don't know how to get the
box. The common code needs subclasses to provide the box parameter.

Another method is to declare 'virtual LayoutBox& box() const = 0' here and let
subclasses implement it. I had such a version but decided to use this version
which has less changed lines.

[Xianzhu, 2015/11/17 18:31:36]

'box()' done.

+
+    void invalidatePaintOfScrollControlsIfNeeded(LayoutBox& ownerBox, const PaintInvalidationState&, const LayoutBoxModelObject& paintInvalidationContainer, LayoutScrollbarPart* scrollCorner, LayoutScrollbarPart* resizer);

[chrishtr, 2015/11/17 00:44:32]

Ditto.

+
+private:
+    void willRemoveScrollbar(Scrollbar*, ScrollbarOrientation) override { ASSERT_NOT_REACHED(); }
+
+    LayoutRect m_horizontalScrollbarPreviousPaintInvalidationRect;
+    LayoutRect m_verticalScrollbarPreviousPaintInvalidationRect;
+    LayoutRect m_scrollCornerPreviousPaintInvalidationRect;
+};
+
+} // namespace blink
+
+#endif // PaintInvalidationCapableScrollableArea_h