Screen flashes black when activating/deactivating main window

Has anyone come across this problem: When I activate/deactivate my application’s main window, the entire screen flashes black for an instant (it’s the same effect as when an old CRT monitor looses sync). This happens both when running in the IDE and in the built application (Windows only of course).

Opening the previous version of my source code (back to 2.3 from 2.4b1) fixes everything, so it’s not related to the version of Xojo/MBS plugins, etc. I develop on Mac OS X, so I just started noticing this. There are certainly quite a few changes from the old version to the new, but nothing obvious that would cause this behavior (absolutely no difference in the window Activate/Deactivate code between the two).

If anyone has seen this before in their app, can you suggest a place to start looking?


-bill k