Both.  Many times I'd like to get rid of the arrow buttons at the ends as they're not always useful and have the slider button be just an oval box with no "grips".  Also, the slider button gets really small when there is a long scroll (it's difficult to grab it with the mouse). 

For a previous program I made a scrollbar that automatically disappears when you don't use it, and I will want to go there with the application I'm working on now. To get it done I had to implement my own Fl_Scroll ...

I can (for now) live with the fixed scrollbars, but I would certainly welcome an alternative version where I can put in my own.

Geert