browser: js exec wait doesn't work with double callbacks

[krichprollsch]

When a callback JS function add new async callback, the JS execution stops w/o waiting for the last async callbacks.

window.addEventListener("load", () => {
  const detailsXHR = new XMLHttpRequest();                                       
  detailsXHR.open('GET', 'http://httpbin.io/json');                                                                      
  detailsXHR.onloadend = function() {                                               
    // the function callback is never called
  };                                                                             
  detailsXHR.send();
});

[krichprollsch]

After further analyze, the listener on window load event is not called back.