Bug 1418456: Yet another followup: Always re-get the style context in the servo case after loading an XBL binding to avoid tripping otherwise useful assertions. r=me

Now we always restyle the whole subtree for Servo, so we may create another
style context for the bound element.

This trips assertions if we happen to create pseudo-element styles for them.

Since that assertion is pretty useful, just re-get the style context all the
time, which is a cheap operation otherwise.

The CLOSED TREE nightmare should end. This wasn't caught in my try run because
another assertion made the crashtests stop running, apparently.

MozReview-Commit-ID: 6U0phWFvvXO

layout/base/nsCSSFrameConstructor.cpp

@@ -2602,11 +2602,15 @@ nsCSSFrameConstructor::ConstructDocElementFrame(Element*                 aDocEle
       mDocument->BindingManager()->AddToAttachedQueue(binding);
     }
 
-    if (resolveStyle) {
-      // FIXME: Should this use ResolveStyleContext?  (The calls in
-      // this function are the only case in nsCSSFrameConstructor
-      // where we don't do so for the construction of a style context
-      // for an element.)
+    if (resolveStyle || styleContext->IsServo()) {
+      // FIXME: Should this use ResolveStyleContext?  (The calls in this
+      // function are the only case in nsCSSFrameConstructor where we don't do
+      // so for the construction of a style context for an element.)
+      //
+      // NOTE(emilio): In the case of Servo, even though resolveStyle returns
+      // false, we re-get the style context to avoid tripping otherwise-useful
+      // assertions when resolving pseudo-elements. Note that this operation in
+      // Servo is cheap.
       styleContext = mPresShell->StyleSet()->ResolveStyleFor(
           aDocElement, nullptr, LazyComputeBehavior::Assert);
       display = styleContext->StyleDisplay();
@@ -5907,11 +5911,13 @@ nsCSSFrameConstructor::AddFrameConstructionItemsInternal(nsFrameConstructorState
         aState.AddPendingBinding(newPendingBinding.forget());
       }
 
-      if (resolveStyle) {
+      // See the comment in the similar-looking block in
+      // ConstructDocElementFrame to see why we always re-fetch the style
+      // context in Servo.
       if (styleContext->IsServo()) {
         styleContext =
           mPresShell->StyleSet()->AsServo()->ResolveServoStyle(aContent->AsElement());
-        } else {
+      } else if (resolveStyle) {
         styleContext =
           ResolveStyleContext(styleContext->AsGecko()->GetParent(),
                               aContent, &aState);
@@ -5919,8 +5925,6 @@ nsCSSFrameConstructor::AddFrameConstructionItemsInternal(nsFrameConstructorState
 
       display = styleContext->StyleDisplay();
       aStyleContext = styleContext;
-      }
-
       aTag = mDocument->BindingManager()->ResolveTag(aContent, &aNameSpaceID);
     }
   }