Do not set default CSS --control-height on multiple select boxes
Inputs are forced to have the same height in all browsers by these rules for the case that they are displayed in a row. For multiple select boxes this does not make much sense, as they are not typically used in a row. It is usually more convenient to have more space to be able to see more options. Applications can override this of course.
The Select2 multiple box was broken due to this rule; the options appeared outside the box.
You can test this with the Extensions/Select2 basic API example; check the last example therein.