The image does not dissolve in the entirety of my screen, a black frame appears, why does this happen?

Can you post an example, preferably a video file or a link to a video on Vimeo,Youtube, etc?

Without actually seeing it my first guess is that OBS (or whatever streaming software you are using) doesn't have it's video resolution set the same as Windows. Please provide more details on your setup, your screens, your computer specs, what version and build of EW, etc.