Bug 1454572: nsComputedDOMStyle: Don't crash when used on a detached pseudo-element. r=emilio,xidorn:emilio

This shouldn't normally happen, but it does in some rare cases; e.g. if an accessibility client queries info for a node that is being removed.

MozReview-Commit-ID: 3nac9ITN66f

--HG--
extra : rebase_source : 238ffc5b14589c91f30f9f0c7d3c23a82914aad3

layout/style/nsComputedDOMStyle.cpp

@@ -501,6 +501,7 @@ nsComputedDOMStyle::DoGetComputedStyleNoFlush(Element* aElement,
                                               StyleType aStyleType)
 {
   MOZ_ASSERT(aElement, "NULL element");
+
   // If the content has a pres shell, we must use it.  Otherwise we'd
   // potentially mix rule trees by using the wrong pres shell's style
   // set.  Using the pres shell from the content also means that any
@@ -521,6 +522,14 @@ nsComputedDOMStyle::DoGetComputedStyleNoFlush(Element* aElement,
     return nullptr;
   }
 
+  if (aElement->IsInNativeAnonymousSubtree() && !aElement->IsInComposedDoc()) {
+    // Normal web content can't access NAC, but Accessibility, DevTools and
+    // Editor use this same API and this may get called for anonymous content.
+    // Computing the style of a pseudo-element that doesn't have a parent doesn't
+    // really make sense.
+    return nullptr;
+  }
+
   // XXX the !aElement->IsHTMLElement(nsGkAtoms::area)
   // check is needed due to bug 135040 (to avoid using
   // mPrimaryFrame). Remove it once that's fixed.