Fix under-invalidation about view background change caused by body style change

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 2a53e6c0bcf239ece950374033a7d277288384c1..27beeecfa72953c51c952fc5c61d46f1705bf98c 100644
--- a/Source/core/layout/LayoutObject.cpp
+++ b/Source/core/layout/LayoutObject.cpp
@@ -1749,6 +1749,13 @@ void LayoutObject::setStyle(PassRefPtr<ComputedStyle> style)
         return;
     }
 
+    // These are used later to check invalidation of document element caused by
+    // body background change. FIXME: This may be related to crbug.com/475115.
+    LayoutObject* oldViewBackgroundLayoutObject = nullptr;
+    LayoutView* layoutView = view();
+    if (RuntimeEnabledFeatures::slimmingPaintEnabled() && layoutView && isBody())
+        oldViewBackgroundLayoutObject = layoutView->backgroundLayoutObject();
+
     StyleDifference diff;
     if (m_style)
         diff = m_style->visualInvalidationDiff(*style);
@@ -1772,6 +1779,18 @@ void LayoutObject::setStyle(PassRefPtr<ComputedStyle> style)
 
     styleDidChange(diff, oldStyle.get());
 
+    if (RuntimeEnabledFeatures::slimmingPaintEnabled() && layoutView && isBody() && layoutView->backgroundLayoutObject() == this) {

[chrishtr, 2015/04/30 01:23:57]

How about always invalidating the LayoutView and the LayoutObject for its
background layout object as a pair, regardless of the reason?

[Xianzhu, 2015/04/30 04:46:39]

We don't need to invalidate the LayoutView, but the documentElementLayoutObject
which may paint the viewport background using body's background style.

Do you mean:
  if (isBody() && shouldDoFullPaintInvalidation()) // caused by style change
     documentElementLayoutObject->setShouldDoFullPaintInvalidation()
?

I'm afraid this would cause unnecessary invalidation of the document element
when e.g.
- the body is animating a style other than background
- the document element has background and the body is animating its own
background

Hopefully the code added in this CL will be removed when we resolve
crbug.com/475115.

[chrishtr, 2015/04/30 17:48:36]

Yeah I mean just always invalidating both if one of them is invalidated. Will it
really be a performance problem?

+        // The rootObject paints view's background. We need to invalidate it when
+        // view's background changes because of change of body's style.
+        if (LayoutObject* rootObject = document().documentElement()->layoutObject()) {
+            if (oldViewBackgroundLayoutObject != this
+                || (!oldStyle && m_style->hasBackground())
+                || (oldStyle && oldStyle->visitedDependentColor(CSSPropertyBackgroundColor) != m_style->visitedDependentColor(CSSPropertyBackgroundColor))
+                || (oldStyle && oldStyle->backgroundLayers() != m_style->backgroundLayers()))
+                rootObject->setShouldDoFullPaintInvalidation();
+        }
+    }
+
     // FIXME: |this| might be destroyed here. This can currently happen for a LayoutTextFragment when
     // its first-letter block gets an update in LayoutTextFragment::styleDidChange. For LayoutTextFragment(s),
     // we will safely bail out with the doesNotNeedLayoutOrPaintInvalidation flag. We might want to broaden