Juan, I've seen this a couple of times on Windows 10. Using MLT's Linear Mask, enacting a Real-Time Preview, somehow the top bar of the RTP disappears so that it can't be closed, minimized, or dragged smaller. You can't scroll to it either, it just seems lost behind the menus/toolbars at top. Unless I'm missing something, it requires a close/reopen to right it. Thanks!