> I would rather think that it is Windows behavior which has something with
> prioritization of the processes? tasks?.

No, it is just the way Fox works (like many other windowed apps). Basically, it makes a mental note to do a repaint real soon now and then continues doing what it was doing anyway - i.e. your lengthy process. But the repainting won't happen until Fox processes events from its queue, which is why inserting a DOEVENTS works admirably for causing invalidated areas to be repainted.

The .Draw method seems to trigger an unconditional repaint immediately (like calling UpdateWindow()), instead of merely marking the window as needing new coat of paint like InvalidateRect() & cousins do.

> I have seen delayed refreshes in other programs too, for example, in
> Windows Explorer.

Gee. That piece of sh*te does not even follow Microsoft's own guidelines and it is certainly not indicative of how a well-written Windows program behaves. And if its evolution from IE4 over W9x and W2K to WXP is any indication than it will only get more b0rken instead of less.