This is a very reasonable demand indeed for websites with lots of facet content, like with product attributes colors for instance.
It can be done with some css, by adding an
overflow: scroll;
(This will add a scrollbar to the facet)
See https://developer.mozilla.org/fr/docs/Web/CSS/overflow
Or, more simply, you could use the select box, or even select2 box, layout on your facet.
