Glitch with SlimSelect dropdown menu position.

[tcompa]

While working on #682, @zonia3000 observed a glitch in how SlimSelect dropdown menu moves around (from "above the button" to "below the button").

For the moment this glitch is fully acceptable, and this issue is just a placeholder in case we want to look into it in the future.

[zonia3000]

I was able to fix this adding a scrollbar inside the modal.

The glitch when using a scrolled modal:

glitch.mp4

The correct behavior when using a modal with a scrollbar inside it:

fixed.mp4