Bug 1611096 - Check webprogresslistener as a fallback for non loaded documents r=rcaliman,ochameau

Depends on D62624

With the previous implementation, an uninitialized document could be returned as a root node.
Here we try to be more explicit and wait for a correct root node. However in some cases a document can remain uninitialized and will never transition to any other state.
If the document is uninitialized but is not currently loading, we should consider it as a valid root node.

Differential Revision: https://phabricator.services.mozilla.com/D62625

devtools/server/actors/inspector/walker.js

@@ -395,12 +395,20 @@ var WalkerActor = protocol.ActorClassWithSpec(walkerSpec, {
   },
 
   _isRootDocumentReady() {
-    if (!this.rootDoc) {
-      return false;
+    if (this.rootDoc) {
+      const { readyState } = this.rootDoc;
+      if (readyState == "interactive" || readyState == "complete") {
+        return true;
+      }
     }
 
-    const { readyState } = this.rootDoc;
-    return readyState == "interactive" || readyState == "complete";
+    // A document might stay forever in unitialized state.
+    // If the target actor is not currently loading a document,
+    // assume the document is ready.
+    const webProgress = this.rootDoc.defaultView.docShell.QueryInterface(
+      Ci.nsIWebProgress
+    );
+    return !webProgress.isLoadingDocument;
   },
 
   /**