Alright, that is a lot to look into. Thanks to all. Guess I'll come back with new problems soon.
Edit- did I say "soon" ? I had a look into the evssimplegraph code today, and restructured bits of my own code accordingly... for instance, the event logic. Puzzled why simplegraph does not seem to use the usual handlers such as OnMouseMove etc., I found that it simply overrides the MouseMove() procedure. Clever. I begin to understand how this works.
But I stumbled across this: When dragging an element (Paintbox) on its parent (Scrollbox) - the drag is logically initiated by the element's mouse down. Ok. But then, when I move the mouse, the event (mouse move or dragover, doesn't matter) goes either to the element or to the parent; thus, if I implement the drag routine in the element, it only works within the bounds of the element; if I implement it in the parent, it only works when the mouse has left the element... my workaround currently is to set enabled to false for all elements during dragging, but I wonder whether this is state of the art.