Bug 1891008 - [geckoview] Delay return of "GeckoViewTabUtil.createNewTab()" until window has finished loading. r=geckoview-reviewers,ohall

The issue arises when attempting to proceed too quickly after
opening a new tab (essentially a new window) before it has
completed loading. This premature action can occasionally
lead to subsequent navigations getting stuck and failing
to reach the "interactive" or "complete" states.

This issue has been observed across numerous WPT tests running
on Android with Fission enabled. With the recent fix,
approximately 14,000 intermittent timeouts have been resolved.

Further investigation into this matter will continue on bug 1837949.

Differential Revision: https://phabricator.services.mozilla.com/D207314

Author: whimboo

mobile/android/modules/geckoview/GeckoViewTestUtils.sys.mjs

@@ -24,12 +24,15 @@ export const GeckoViewTabUtil = {
         if (subject.name === sessionId) {
           Services.obs.removeObserver(
             openingObserver,
-            "geckoview-window-created"
+            "browser-delayed-startup-finished"
           );
           resolve(subject);
         }
       };
-      Services.obs.addObserver(openingObserver, "geckoview-window-created");
+      Services.obs.addObserver(
+        openingObserver,
+        "browser-delayed-startup-finished"
+      );
     });
 
     try {