Annoying Black screen on gallery mode

[TCH]

Just participated in a meeting and during the meeting I had the persons shared screen on one of my screens, the video in gallery mode on my surface pro and the chat screen on a different screen, when I would rest my mouse on the gallery view a Black rectangle would appear after a bit and block half the screen, once I touched or moved the mouse the rectangle would go away and then come back again once I rested the mouse, what was causing this or how do you find or get rid of that rectangle?