Bug 1670848: Handling aborting history navigations when others run r=smaug

Handles edge cases around history.go() with one event loop spin

Differential Revision: https://phabricator.services.mozilla.com/D93302

docshell/base/nsDocShell.cpp

@@ -11090,7 +11090,17 @@ nsresult nsDocShell::UpdateURLAndHistory(Document* aDocument, nsIURI* aNewURI,
     // Step 2.
 
     // Step 2.2, "Remove any tasks queued by the history traversal task
-    // source..."
+    // source that are associated with any Document objects in the
+    // top-level browsing context's document family."  This is very hard in
+    // SessionHistoryInParent since we can't synchronously access the
+    // pending navigations that are already sent to the parent. We can
+    // abort any AsyncGo navigations that are waiting to be sent.  If we
+    // send a message to the parent, it would be processed after any
+    // navigations previously sent.  So long as we consider the "history
+    // traversal task source" to be the list in this process we match the
+    // spec.  If we move the entire list to the parent, we can handle the
+    // aborting of loads there, but we don't have a way to synchronously
+    // remove entries as we do here for non-SHIP.
     RefPtr<ChildSHistory> shistory = GetRootSessionHistory();
     if (shistory) {
       shistory->RemovePendingHistoryNavigations();