Bug 1492327 - Mimic XUL activeElement behavior for XUL in XHTML. r=smaug

Return the document element as the activeElement when there is no body
element, the document is chrome privileged, and the document element
is a XUL element.

MozReview-Commit-ID: JFDLAqOmLTS

Differential Revision: https://phabricator.services.mozilla.com/D6448

--HG--
extra : moz-landing-system : lando

browser/base/content/tabbrowser.js

@@ -1106,9 +1106,7 @@ window._gBrowser = {
         newBrowser._urlbarFocused &&
         gURLBar &&
         gURLBar.focused;
-      // In an HTML document (built with MOZ_BROWSER_XHTML), the activeElement
-      // can be null, so check before attempting to blur it (Bug 1485157).
-      if (!keepFocusOnUrlBar && document.activeElement) {
+      if (!keepFocusOnUrlBar) {
         // Clear focus so that _adjustFocusAfterTabSwitch can detect if
         // some element has been focused and respect that.
         document.activeElement.blur();

dom/base/nsDocument.cpp

@@ -3586,9 +3586,22 @@ nsIDocument::GetActiveElement()
 
   // No focused element anywhere in this document.  Try to get the BODY.
   if (IsHTMLOrXHTML()) {
+    Element* bodyElement = AsHTMLDocument()->GetBody();
+    if (bodyElement) {
+      return bodyElement;
+    }
+    // Special case to handle the transition to browser.xhtml where there is
+    // currently not a body element, but we need to match the XUL behavior.
+    // This should be removed when bug 1492582 is resolved.
+    if (nsContentUtils::IsChromeDoc(this)) {
+      Element* docElement = GetDocumentElement();
+      if (docElement && docElement->IsXULElement()) {
+        return docElement;
+      }
+    }
     // Because of IE compatibility, return null when html document doesn't have
     // a body.
-    return AsHTMLDocument()->GetBody();
+    return nullptr;
   }
 
   // If we couldn't get a BODY, return the root element.