CSS: Avoid invalidating style when only descendants need updating

Source/core/css/invalidation/InvalidationSet.cpp

Index: Source/core/css/invalidation/InvalidationSet.cpp
diff --git a/Source/core/css/invalidation/InvalidationSet.cpp b/Source/core/css/invalidation/InvalidationSet.cpp
index 617e9b974e5a946d7dfabd2307c2c5c4e39b0b1e..426f23ddc02181a3eb06057a55c4c0a3d578b618 100644
--- a/Source/core/css/invalidation/InvalidationSet.cpp
+++ b/Source/core/css/invalidation/InvalidationSet.cpp
@@ -52,7 +52,8 @@ void InvalidationSet::cacheTracingFlag()
 }
 
 InvalidationSet::InvalidationSet()
-    : m_allDescendantsMightBeInvalid(false)
+    : m_appliesDirectly(false)
+    , m_allDescendantsMightBeInvalid(false)
     , m_customPseudoInvalid(false)
     , m_treeBoundaryCrossing(false)
     , m_insertionPointCrossing(false)
@@ -98,6 +99,9 @@ bool InvalidationSet::invalidatesElement(Element& element) const
 
 void InvalidationSet::combine(const InvalidationSet& other)
 {
+    if (other.appliesDirectly())
+        setAppliesDirectly();

[rune, 2015/09/16 08:44:32]

The existing code did an early return on wholeSubtreeInvalid() before copying
any other bits. Did you put this before on purpose? If not, you can move it down
where the other bits are copied.

[Eric Willigers, 2015/09/16 13:32:08]

Moved.  The original positioning, and the original naming, was motivated by
sibling invalidation: I was hoping to avoid allocating a sibling descendant
invalidation set if only the sibling itself is being invalidated. Not important:
it is clearer to instead have a sibling descendant invalidation set that is
empty and has 'invalidatesSelf'.

+
     // No longer bother combining data structures, since the whole subtree is deemed invalid.
     if (wholeSubtreeInvalid())
         return;