Tests won't close the browser window after the test since we updated to 18.0

Our tests were working just fine but since we updated to 18.0, our tests sometimes

don't close our browser window and the open agent gives a message:

 The last browser action could not yet be completed. The website is busy because of the following reason(s):
 One or more Javascript timers did not expire. Use option OPT_XBROWSER_TIMER_SYNC_MODE to turn off timer sync.

We are now trying to set this OPT_XBROWSER_TIMER_SYNC_MODE:

  desktop.setOption(CommonOptions.OPT_XBROWSER_TIMER_SYNC_MODE, ????);

but there are no values

documented. Does anyone know how this modes work and what they do?

thx