Don't paint empty scrollbar parts

Source/core/paint/ScrollableAreaPainter.cpp

 // Copyright 2014 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.
 
 #include "config.h"
 #include "core/paint/ScrollableAreaPainter.h"
 
 #include "core/layout/LayoutView.h"
 #include "core/page/Page.h"
 #include "core/paint/DeprecatedPaintLayer.h"
 #include "core/paint/DeprecatedPaintLayerScrollableArea.h"
 #include "core/paint/ScrollbarPainter.h"
 #include "core/paint/TransformRecorder.h"
 #include "platform/graphics/GraphicsContext.h"
 #include "platform/graphics/GraphicsContextStateSaver.h"
 #include "platform/graphics/paint/DrawingRecorder.h"
 
 namespace blink {
 
 void ScrollableAreaPainter::paintResizer(GraphicsContext* context, const IntPoint& paintOffset, const IntRect& damageRect)
 {
     if (m_scrollableArea.box().style()->resize() == RESIZE_NONE)
         return;
 
     IntRect absRect = m_scrollableArea.resizerCornerRect(m_scrollableArea.box().pixelSnappedBorderBoxRect(), ResizerForPointer);
+    if (absRect.isEmpty())
+        return;
     absRect.moveBy(paintOffset);
-    if (!absRect.intersects(damageRect))
-        return;
 
     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;
 
     drawPlatformResizerImage(context, absRect);
 
     // Draw a frame around the resizer (1px grey line) if there are any scrollbars present.
     // Clipping will exclude the right and bottom edges of this frame.
     if (!m_scrollableArea.hasOverlayScrollbars() && m_scrollableArea.hasScrollbar()) {
         GraphicsContextStateSaver stateSaver(*context);
@@ skipping 115 matching lines @@
     if (m_scrollableArea.resizerCornerRect(borderBox, ResizerForPointer).intersects(localRect))
         return true;
 
     return false;
 }
 
 
 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()) {
         if (!absRect.intersects(damageRect))
             return;
         ScrollbarPainter::paintIntoRect(m_scrollableArea.scrollCorner(), context, paintOffset, LayoutRect(absRect));
         return;
     }
 
     if (!RuntimeEnabledFeatures::slimmingPaintEnabled() && !absRect.intersects(damageRect))
         return;
 
     // We don't want to paint white if we have overlay scrollbars, since we need
     // to see what is behind it.
     if (m_scrollableArea.hasOverlayScrollbars())
         return;
 
     DrawingRecorder recorder(*context, m_scrollableArea.box(), DisplayItem::ScrollbarCorner, absRect);
     if (recorder.canUseCachedDrawing())
         return;
 
     context->fillRect(absRect, Color::white);
 }
 
 } // namespace blink