[S.P.] Invalidate the document element for view background when needed

Source/core/layout/LayoutObject.cpp

Index: Source/core/layout/LayoutObject.cpp
diff --git a/Source/core/layout/LayoutObject.cpp b/Source/core/layout/LayoutObject.cpp
index 2fa594d795b58033d5ff04abe59b35de5c51df22..f771733345787ff5af7dbf5da53be6e53e899ad6 100644
--- a/Source/core/layout/LayoutObject.cpp
+++ b/Source/core/layout/LayoutObject.cpp
@@ -628,6 +628,12 @@ bool LayoutObject::skipInvalidationWhenLaidOutChildren() const
     if (hasNonCompositedScrollbars())
         return false;
 
+    // The document element always draws background even if it has no background specified.
+    // In non-slimming-paint mode, the invalidation rect of the LayoutView covers the area

[chrishtr, 2015/03/18 19:30:05]

I guess it's hard to do that invalidation on the document?

Maybe we should have a LayoutDocument subclass or something, to hande the
various isDocumentElement() calls sprinked throughout layout and paint.

[Xianzhu, 2015/03/18 19:36:28]

Will explore this way.

[Xianzhu, 2015/03/18 22:44:18]

This seems not feasible because the layout object for document element may be of
different types, e.g. LayoutBlockFlow or SVGRoot or others depending on the
document type.

+    // needing paint invalidation.
+    if (RuntimeEnabledFeatures::slimmingPaintEnabled() && isDocumentElement())
+        return false;
+
     return rendererHasNoBoxEffect();
 }