I've now tried two different grids: itGrid and Spread. Under some circumstances - and I've not been able to pin down the circumstances - neither of them like redrawing once the window gets above a certain area. Instead the area inside the control is never redrawn, and retains its previous content. If the window gets below that size, it starts redrawing again.
I'm running a dual-head 1280x1024 system, and can sometimes cause this behaviour by sizing the control to over c. 75% of the area. However, it appears that it can *also* be caused by having many smaller windows containing the control.
Once this behaviour starts on a machine, it appears to be impossible to stop. However, it doesn't necessarily start immediately after installing the app - or, indeed, at all. Quitting and restarting the app may help, as may rebooting the machine, but neither seems to provide a permanent solution.
Windows 2000 SP4, VB6 SP6, itGrid v2.0.0 (written in VB), Spread v7.0 (written in C++), windows inside MDI.
All ideas welcome!