Don't paint empty scrollbar parts

Source/platform/scroll/ScrollbarTheme.cpp

 /*
  * Copyright (C) 2011 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 32 matching lines @@
 
 #if !OS(MACOSX)
 #include "public/platform/WebRect.h"
 #include "public/platform/WebThemeEngine.h"
 #endif
 
 namespace blink {
 
 bool ScrollbarTheme::gMockScrollbarsEnabled = false;
 
+static inline bool shouldPaintScrollbarPart(const IntRect& partRect, const IntRect& damageRect)
+{
+    return (RuntimeEnabledFeatures::slimmingPaintEnabled() && !partRect.isEmpty()) || damageRect.intersects(partRect);
+}
+
 bool ScrollbarTheme::paint(ScrollbarThemeClient* scrollbar, GraphicsContext* graphicsContext, const IntRect& damageRect)
 {
     // Create the ScrollbarControlPartMask based on the damageRect
     ScrollbarControlPartMask scrollMask = NoPart;
 
     IntRect backButtonStartPaintRect;
     IntRect backButtonEndPaintRect;
     IntRect forwardButtonStartPaintRect;
     IntRect forwardButtonEndPaintRect;
     if (hasButtons(scrollbar)) {
         backButtonStartPaintRect = backButtonRect(scrollbar, BackButtonStartPart, true);
-        if (RuntimeEnabledFeatures::slimmingPaintEnabled() || damageRect.intersects(backButtonStartPaintRect))
+        if (shouldPaintScrollbarPart(backButtonStartPaintRect, damageRect))
             scrollMask |= BackButtonStartPart;
         backButtonEndPaintRect = backButtonRect(scrollbar, BackButtonEndPart, true);
-        if (RuntimeEnabledFeatures::slimmingPaintEnabled() || damageRect.intersects(backButtonEndPaintRect))
+        if (shouldPaintScrollbarPart(backButtonEndPaintRect, damageRect))
             scrollMask |= BackButtonEndPart;
         forwardButtonStartPaintRect = forwardButtonRect(scrollbar, ForwardButtonStartPart, true);
-        if (RuntimeEnabledFeatures::slimmingPaintEnabled() || damageRect.intersects(forwardButtonStartPaintRect))
+        if (shouldPaintScrollbarPart(forwardButtonStartPaintRect, damageRect))
             scrollMask |= ForwardButtonStartPart;
         forwardButtonEndPaintRect = forwardButtonRect(scrollbar, ForwardButtonEndPart, true);
-        if (RuntimeEnabledFeatures::slimmingPaintEnabled() || damageRect.intersects(forwardButtonEndPaintRect))
+        if (shouldPaintScrollbarPart(forwardButtonEndPaintRect, damageRect))
             scrollMask |= ForwardButtonEndPart;
     }
 
     IntRect startTrackRect;
     IntRect thumbRect;
     IntRect endTrackRect;
     IntRect trackPaintRect = trackRect(scrollbar, true);
     if (RuntimeEnabledFeatures::slimmingPaintEnabled() || damageRect.intersects(trackPaintRect))
         scrollMask |= TrackBGPart;
     bool thumbPresent = hasThumb(scrollbar);
     if (thumbPresent) {
         IntRect track = trackRect(scrollbar);
         splitTrack(scrollbar, track, startTrackRect, thumbRect, endTrackRect);
-        if (RuntimeEnabledFeatures::slimmingPaintEnabled() || damageRect.intersects(thumbRect))
+        if (shouldPaintScrollbarPart(thumbRect, damageRect))
             scrollMask |= ThumbPart;
-        if (RuntimeEnabledFeatures::slimmingPaintEnabled() || damageRect.intersects(startTrackRect))
+        if (shouldPaintScrollbarPart(startTrackRect, damageRect))
             scrollMask |= BackTrackPart;
-        if (RuntimeEnabledFeatures::slimmingPaintEnabled() || damageRect.intersects(endTrackRect))
+        if (shouldPaintScrollbarPart(endTrackRect, damageRect))
             scrollMask |= ForwardTrackPart;
     }
 
     // Paint the scrollbar background (only used by custom CSS scrollbars).
     paintScrollbarBackground(graphicsContext, scrollbar);
 
     // Paint the back and forward buttons.
     if (scrollMask & BackButtonStartPart)
         paintButton(graphicsContext, scrollbar, backButtonStartPaintRect, BackButtonStartPart);
     if (scrollMask & BackButtonEndPart)
@@ skipping 265 matching lines @@
         return DisplayItem::ScrollbarBackTrack;
     case ForwardTrackPart:
         return DisplayItem::ScrollbarForwardTrack;
     default:
         ASSERT_NOT_REACHED();
         return DisplayItem::ScrollbarBackTrack;
     }
 }
 
 } // namespace blink