x.ontimeout = function(){
    try{opt.aftersend()}catch(e){/*ignore*/}
    opt.onerror(new Error("XHR timeout of "+x.timeout+"ms expired."));
  };
  x.onreadystatechange = function(){
    if(XMLHttpRequest.DONE !== x.readyState) return;
    try{opt.aftersend()}catch(e){/*ignore*/}
    if(false && 0===x.status){
      /* For reasons unknown, we _sometimes_ trigger x.status==0 in FF
         when the /chat page starts up, but not in Chrome nor in other
         apps. Insofar as has been determined, this happens before a
         request is actually sent and it appears to have no
         side-effects on the app other than to generate an error
         (i.e. no requests/responses are missing). This is a silly
         workaround which may or may not bite us later. If so, it can
         be removed at the cost of an unsightly console error message
         in FF. */
      return;
    }
    if(200!==x.status){
      let err;
      try{
        const j = JSON.parse(x.response);
        if(j.error) err = new Error(j.error);
      }catch(ex){/*ignore*/}
      opt.onerror(err || new Error("HTTP response status "+x.status+"."));