Don't paint empty scrollbar parts

Source/core/paint/ScrollableAreaPainter.cpp

Index: Source/core/paint/ScrollableAreaPainter.cpp
diff --git a/Source/core/paint/ScrollableAreaPainter.cpp b/Source/core/paint/ScrollableAreaPainter.cpp
index d2ba3197894c23924ebd5bc7699fa46b04b245f5..6cfb3fbaa93a7def7c1457e78ffe77ba658b707c 100644
--- a/Source/core/paint/ScrollableAreaPainter.cpp
+++ b/Source/core/paint/ScrollableAreaPainter.cpp
@@ -23,15 +23,20 @@ void ScrollableAreaPainter::paintResizer(GraphicsContext* context, const IntPoin
         return;
 
     IntRect absRect = m_scrollableArea.resizerCornerRect(m_scrollableArea.box().pixelSnappedBorderBoxRect(), ResizerForPointer);
-    absRect.moveBy(paintOffset);
-    if (!absRect.intersects(damageRect))
+    if (absRect.isEmpty())
         return;
+    absRect.moveBy(paintOffset);
 
     if (m_scrollableArea.resizer()) {
+        if (!absRect.intersects(damageRect))

[chrishtr, 2015/06/05 22:50:09]

Why is this ok? I thought we had to paint all or none now?

[Xianzhu, 2015/06/05 22:56:54]

We paint all or none for non-custom parts. For custom parts, we paint them as
normal LayoutBlocks which use the custom layout objects as display item clients.

[chrishtr, 2015/06/05 22:59:11]

OK, then...

+            return;
         ScrollbarPainter::paintIntoRect(m_scrollableArea.resizer(), context, paintOffset, LayoutRect(absRect));
         return;
     }
 
+    if (!RuntimeEnabledFeatures::slimmingPaintEnabled() && !absRect.intersects(damageRect))

[chrishtr, 2015/06/05 22:59:11]

Why condition on slimming paint here?

[Xianzhu, 2015/06/05 23:04:41]

This is the non-custom path. In slimming paint mode, we need to paint the whole
part regardless of damageRect.

(This has the same logic as paintScrollCorner. I missed this method in
https://codereview.chromium.org/1142283006/).

+        return;
+
     DrawingRecorder recorder(*context, m_scrollableArea.box(), DisplayItem::Resizer, absRect);
     if (recorder.canUseCachedDrawing())
         return;
@@ -167,6 +172,8 @@ bool ScrollableAreaPainter::overflowControlsIntersectRect(const IntRect& localRe
 void ScrollableAreaPainter::paintScrollCorner(GraphicsContext* context, const IntPoint& paintOffset, const IntRect& damageRect)
 {
     IntRect absRect = m_scrollableArea.scrollCornerRect();
+    if (absRect.isEmpty())
+        return;
     absRect.moveBy(paintOffset);
 
     if (m_scrollableArea.scrollCorner()) {