The current implementation of datebox calendar open() creates a timeout such as:

zul.db.CalendarPop.open(){
...
		setTimeout(function () {
			_reposition(db, silent);
			zWatch.fireDown('onVParent', self.parent.$n('pp'), { shadow: self._shadow });
		}, 150);

This causes flickering when the popup re-position itself in some cases.
It might be possible to avoid calling the arbitrary timeout at 150ms by using zWatch onCommandReady if this need to be executed later.