Skip PaintPhaseDescendantBlockBackgroundsOnly phase if possible

third_party/WebKit/Source/core/layout/LayoutBox.cpp

Index: third_party/WebKit/Source/core/layout/LayoutBox.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBox.cpp b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
index d0c904e9c63fda921f0c2b284ff6d27d49e23845..f4abadca4ff06aa89872c24a1c3fb1b0f348ad8c 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBox.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
@@ -1471,6 +1471,12 @@ PaintInvalidationReason LayoutBox::invalidatePaintIfNeeded(PaintInvalidationStat
     if (isFloating())
         paintInvalidationState.enclosingSelfPaintingLayer(*this).setNeedsPaintPhaseFloat();
 
+    if (hasBoxDecorationBackground() || (hasOverflowClip() && scrollableArea()->hasOverflowControls())) {

[chrishtr, 2016/02/01 17:43:49]

Why the is this logic the right logic? In particular the part about controls and
overflow clip. Please also add a comment explaining.

[Xianzhu, 2016/02/01 17:58:39]

Added comment:
// We also paint overflow controls in background phase.

+        PaintLayer& layer = paintInvalidationState.enclosingSelfPaintingLayer(*this);
+        if (layer.layoutObject() != this)
+            layer.setNeedsPaintPhaseDescendantBlockBackgrounds();
+    }
+
     PaintInvalidationReason fullInvalidationReason = fullPaintInvalidationReason();
     // If the current paint invalidation reason is PaintInvalidationDelayedFull, then this paint invalidation can delayed if the
     // LayoutBox in question is not on-screen. The logic to decide whether this is appropriate exists at the site of the original