I never said a hack was something bad. Especially if there are no other options, a hack is obviously good because it's the only way to do what you want. If there are other options that give you the exact same behavior with 'better', more conventional methods, then of course the hack is not recommended, but that doesn't seem to be the case here.

I only call it a hack because it is a 'work around' for something that is impossible (painting on a webbrowser).

As for unreliable things with this method, I can't really think of anything, but that doesn't mean there aren't any. That's the "danger" of these things: you can't really be sure that it will work 100% of the time. The fact remains that there is no 'hard' link between the 'painting form' and the underlying webbrowser. If you close all loop holes, then theoretically you could mimic such a link but you can't be sure if you know all loop holes. Of course, with sufficient testing you can make sure that nothing will go wrong in most of the cases. I've seen you recommend this method for various other things too, so I expect it will be tested sufficiently by now so it should all be fine.

I guess what I'm trying to say is that windows doesn't know about your connection between these two forms, so there is no reason to assume that it will go right. I can imagine windows hiding one of the windows, for some reason, while not hiding the other. It tends to do stuff like that once in a while