Hey
There are a few questions to help me understand what is happening in your case. When you say the UE window are you referring to the Editor window or a game window (such as the standalone window). If this is a game window does this apply to a packaged version of the game or when playing in the editor? Also, what window are you parenting the UE window to? Do you have similar behavior if you try to reparent the window for another program to a different window?
Cheers