Bug 1511138 - Never return styles for disconnected elements. r=smaug

https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle says:

> If elt is connected, part of the flat tree, and its shadow-including root...

WebKit and Blink already do this, and we do it already except for cross-document
situations, where we can end up with a PresShell even if GetPresShellForContent
returns null.

The style system should be able to rely on ShadowRoots having a non-null shadow
host.

Differential Revision: https://phabricator.services.mozilla.com/D13472

layout/style/nsComputedDOMStyle.cpp

@@ -508,11 +508,12 @@ already_AddRefed<ComputedStyle> nsComputedDOMStyle::DoGetComputedStyleNoFlush(
     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.
+  if (!aElement->IsInComposedDoc()) {
+    // Don't return styles for disconnected elements, that makes no sense. This
+    // can only happen with a non-null presShell for cross-document calls.
+    //
+    // FIXME(emilio, bug 1483798): This should also not return styles for
+    // elements outside of the flat tree, not just outside of the document.
     return nullptr;
   }