Hello, awesome FLTK devs!

I find myself needing to get notified and run custom code when scrolling is happening / stops. Currently there seems no way to achieve that. Perhaps it would be generally useful if Fl_Scroll had callback(s) triggered when its x/yposition changes.

Background: I am making a file explorer for pictures, displaying thumbnails. For performance reasons I only show thumbnails of pictures in the visible viewport. When the user scrolls to a new position (or is scrolling slowly), I want to load and show also these newly visible pictures' thumbnails. Thus I want to trigger the image loading function when scrolling ends.

Obviously FLTK lived fine without this for many years and I was able to implement this by querying the scroll's yposition every 100ms, so it might not be worth the effort. But it would be nice.

Best regards, Jakub Holy