Speeding up readPixels

Questions
8

Strange… In _clientWaitAsync…

_clientWaitAsync = function(sync, flags = 0, interval_ms = 10) {
  return new Promise((resolve, reject) => {
    let check = () => {
      const res = gl.clientWaitSync(sync, flags, 0);
      if (res == gl.WAIT_FAILED) {
        console.log(gl.getError());
        reject();
        return;
      }
      if (res == gl.TIMEOUT_EXPIRED) {
        setTimeout(check, interval_ms);
        return;
      }
      resolve();
    };
    check();
  });
}

gl.clientWaitSync is returning WAIT_FAILED on Firefox, but works fine on Chrome.

gl.getError() is returning 0 (no error), and everything seems to be working fine if I just ignore the error and change reject() into resolve(). What could have caused the WAIT_FAILED?