Implement the ::backdrop pseudo-element for modal <dialog>.

Source/core/css/resolver/StyleResolver.cpp

Index: Source/core/css/resolver/StyleResolver.cpp
diff --git a/Source/core/css/resolver/StyleResolver.cpp b/Source/core/css/resolver/StyleResolver.cpp
index b09bfc53c46a875974642defef39faee47f7396d..1635e21d747f8a32f4e92e7855c583ed4d07808e 100644
--- a/Source/core/css/resolver/StyleResolver.cpp
+++ b/Source/core/css/resolver/StyleResolver.cpp
@@ -1273,7 +1273,7 @@ PassRefPtr<RenderStyle> StyleResolver::pseudoStyleForElement(Element* e, const P
 
     state.initForStyleResolve(document(), e, parentStyle);
 
-    if (m_state.parentStyle()) {
+    if (pseudoStyleRequest.allowsInheritance(m_state.parentStyle())) {
         state.setStyle(RenderStyle::create());
         state.style()->inheritFrom(m_state.parentStyle());
     } else {
@@ -1447,13 +1447,19 @@ static EDisplay equivalentBlockDisplay(EDisplay display, bool isFloating, bool s
 // CSS requires text-decoration to be reset at each DOM element for tables,
 // inline blocks, inline tables, run-ins, shadow DOM crossings, floating elements,
 // and absolute or relatively positioned elements.
-static bool doesNotInheritTextDecoration(RenderStyle* style, Element* e)
+static bool doesNotInheritTextDecoration(const RenderStyle* style, const Element* e)
 {
     return style->display() == TABLE || style->display() == INLINE_TABLE || style->display() == RUN_IN
         || style->display() == INLINE_BLOCK || style->display() == INLINE_BOX || isAtShadowBoundary(e)
         || style->isFloating() || style->hasOutOfFlowPosition();
 }
 
+// Helper function since we can't use element->isInTopLayer() when element is null.
+static bool isInTopLayer(const Element* element, const RenderStyle* style)
+{
+    return (element && element->isInTopLayer()) || (style && style->styleType() == BACKDROP);

[esprehn, 2013/07/04 08:34:07]

If the backdrop is in the top layer, why do you need the BACKDROP check? You
should just remove this method, e && e->isInTopLayer() should work fine since
when you resolve for ::backdrop the parentElement() of the PseudoElement (the
"e" in here) should be isInTopLayer().

[falken, 2013/07/04 09:06:35]

createPseudoElementIfNeeded asks for the backdrop's style,
which brings us to StyleResolver::pseudoStyleForElement which
calls adjustRenderStyle with e = 0. So if it's just
e && e->isInTopLayer(), it won't be considered in the top
layer. Once the ::backdrop is created and attached, it keeps
that bad style.

I would think it gets a style recalc when it gets
added to the top layer though, since that forces a reattach.
Will investigate.

Here is a full stack trace:
#0  WebCore::isRenderedInTopLayer (style=0x3cd41e969650, e=0x0) at
../../third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp:1460
#1  0x0000000002d4de2c in WebCore::StyleResolver::adjustRenderStyle
(this=0x3cd41e8886a0, style=0x3cd41e969650, parentStyle=0x3cd41e88a180, e=0x0)
    at ../../third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp:1532
#2  0x0000000002d4cef5 in WebCore::StyleResolver::pseudoStyleForElement
(this=0x3cd41e8886a0, e=0x356f76df0028, pseudoStyleRequest=...,
parentStyle=0x3cd41e851d30)
    at ../../third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp:1305
#3  0x0000000002ac204e in WebCore::RenderObject::getUncachedPseudoStyle
(this=0x3cd41e954038, pseudoStyleRequest=..., parentStyle=0x3cd41e851d30,
ownStyle=0x0)
    at ../../third_party/WebKit/Source/core/rendering/RenderObject.cpp:2806
#4  0x0000000002ac1da0 in WebCore::RenderObject::getCachedPseudoStyle
(this=0x3cd41e954038, pseudo=WebCore::BACKDROP, parentStyle=0x0) at
../../third_party/WebKit/Source/core/rendering/RenderObject.cpp:2776
#5  0x00000000021287a9 in WebCore::Element::createPseudoElementIfNeeded
(this=0x356f76df0028, pseudoId=WebCore::BACKDROP) at
../../third_party/WebKit/Source/core/dom/Element.cpp:2435
#6  0x000000000212450f in WebCore::Element::attach (this=0x356f76df0028,
context=...) at ../../third_party/WebKit/Source/core/dom/Element.cpp:1354
#7  0x0000000001c91f07 in WebCore::Node::reattach (this=0x356f76df0028,
context=...) at ../../third_party/WebKit/Source/core/dom/Node.h:919
#8  0x0000000002124f16 in WebCore::Element::recalcStyle (this=0x356f76df0028,
change=WebCore::Node::NoChange) at
../../third_party/WebKit/Source/core/dom/Element.cpp:1485
#9  0x0000000002125398 in WebCore::Element::recalcStyle (this=0x356f76dd9dd0,
change=WebCore::Node::NoChange) at
../../third_party/WebKit/Source/core/dom/Element.cpp:1552
#10 0x0000000002125398 in WebCore::Element::recalcStyle (this=0x356f76dd9d00,
change=WebCore::Node::NoChange) at
../../third_party/WebKit/Source/core/dom/Element.cpp:1552
#11 0x00000000020ca14c in WebCore::Document::recalcStyle (this=0x356f76dd4028,
change=WebCore::Node::NoChange) at
../../third_party/WebKit/Source/core/dom/Document.cpp:1706
#12 0x00000000020ca49e in WebCore::Document::updateStyleIfNeeded
(this=0x356f76dd4028) at
../../third_party/WebKit/Source/core/dom/Document.cpp:1753
#13 0x00000000020ca66b in WebCore::Document::updateLayout (this=0x356f76dd4028)
at ../../third_party/WebKit/Source/core/dom/Document.cpp:1782
#14 0x00000000020ca850 in
WebCore::Document::updateLayoutIgnorePendingStylesheets (this=0x356f76dd4028) at
../../third_party/WebKit/Source/core/dom/Document.cpp:1826
#15 0x000000000415c9e3 in WebCore::HTMLDialogElement::reposition
(this=0x356f76df0028) at
../../third_party/WebKit/Source/core/html/HTMLDialogElement.cpp:84
#16 0x000000000415cc68 in WebCore::HTMLDialogElement::showModal
(this=0x356f76df0028, ec=@0x7fff02595cbc: 0) at
../../third_party/WebKit/Source/core/html/HTMLDialogElement.cpp:119

[falken, 2013/07/04 12:18:01]

OK I see now. The problem is just that
StyleResolver::pseudoStyleForElement calls
adjustRenderStyle(state.style(), m_state.parentStyle(), 0);
So adjustRenderStyle for backdrop will always have null "e".

I updated the helper function with a FIXME comment.
I'll try to fix it as part of making adjustRenderStyle
smarter for pseudoelements.

+}
+
 static bool isDisplayFlexibleBox(EDisplay display)
 {
     return display == FLEX || display == INLINE_FLEX;
@@ -1522,7 +1528,7 @@ void StyleResolver::adjustRenderStyle(RenderStyle* style, RenderStyle* parentSty
             style->setDisplay(BLOCK);
 
         // Per the spec, position 'static' and 'relative' in the top layer compute to 'absolute'.
-        if (e && e->isInTopLayer() && (style->position() == StaticPosition || style->position() == RelativePosition))
+        if (isInTopLayer(e, style) && (style->position() == StaticPosition || style->position() == RelativePosition))
             style->setPosition(AbsolutePosition);
 
         // Absolute/fixed positioned elements, floating elements and the document element need block-like outside display.
@@ -1578,7 +1584,7 @@ void StyleResolver::adjustRenderStyle(RenderStyle* style, RenderStyle* parentSty
         || style->hasBlendMode()
         || style->position() == StickyPosition
         || (style->position() == FixedPosition && e && e->document()->page() && e->document()->page()->settings()->fixedPositionCreatesStackingContext())
-        || (e && e->isInTopLayer())
+        || isInTopLayer(e, style)
         ))
         style->setZIndex(0);