How to make a scroll bar visible or show in a sidepanel when sidepanel's childcomponents's position over the height

I add some sliders in a sidepanel, but the scroll bar doesn’t show when slider’s position over the height. Is there any way to make the scroll bar show?
By the way, how to make a slider’s textentrybox in the middle of the slider when the sliderstyle is IncDecButtons just like this:
This is the effect I want.