That is truly odd. I did the same exact thing and it doesn't happen on mine.
This time when it happens, minimize the main window or just open another window on top of it. When you do that and then go back to it Windows updates and redrawns the forms. I noticed when you mouse over it they come back as they are drawing themselves.
I am at a loss to why the settopmost api is doing that in the program.
I can fix any bug, as long as I can replicate it, so far I cant.
Shane