Reland "Implement FullScreen using top layer."

third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp

Index: third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
diff --git a/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp b/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
index 21f77974b4d1972111d3706080b499877612f2c9..d98cbd86379dc88a892b72ac056d17a23686a3f3 100644
--- a/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
+++ b/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
@@ -887,7 +887,10 @@ PseudoElement* StyleResolver::createPseudoElementIfNeeded(Element& parent, Pseud
     if (pseudoId == PseudoIdFirstLetter && (parent.isSVGElement() || !FirstLetterPseudoElement::firstLetterTextLayoutObject(parent)))
         return nullptr;
 
-    if (!canHaveGeneratedChildren(*parentLayoutObject))
+    // The backdrop pseudo element generates a new stacking context and its
+    // layout object does not become a child of |parentLayoutObject|. The
+    // exemption is needed so that replaced content also gets a backdrop.
+    if (pseudoId != PseudoIdBackdrop && !canHaveGeneratedChildren(*parentLayoutObject))

[foolip, 2016/07/07 16:11:55]

The comment is written with confidence, but please don't assume that it's
entirely true, I'm concerned that this may actually still be dangerous for some
case I haven't thought about.

[eae, 2016/07/12 17:09:49]

Can we add asserts to ensure that our assumptions here hold true?

[foolip, 2016/07/12 20:19:48]

Good, idea, at some point it should be possible to assert that the layout object
created for the backdrop doesn't become the child of a layout object for which
canHaveGeneratedChildren returns true. I'll look into it tomorrow.

         return nullptr;
 
     ComputedStyle* parentStyle = parentLayoutObject->mutableStyle();