The boxes were part of my test program, see the post that I referred to in my previous answer ("Problem with redraw() / draw()" from mid august, I already moved on and don't have it on my computer anymore). You can plug in the code for the modified scroll_B class to see what it does. The crux is that you provide your own callback function for the scrollbar. It should of course perform the task of the original callback which is just positioning the scroll:

  Fl_Scroll* s = (Fl_Scroll*)(o->parent());
  s->scroll_to(int(((Fl_Scrollbar*)o)->value()), s->yposition());

(This is the only thing it does) and then you can have it perform the tasks specific for your application. 

On a general note: I find the Fl_Scroll class not very adaptable to my needs, very un-FLTK-like. I hope the development team can come up with an "improved" version. I do greatly appreciate all the work that is going into FLTK, it is quite a feat! A great thanks to everyone!