Whenever you drag an application to the top, bottom, or corners of a screen, a blurred overlay appears showing you the area it's being assigned to. I inadvertently experienced this when I locked my computer to walk away from it for a few minutes, blindly smashing the Win+L key (and likely other keys around it) until the screen locked. When I came back to my computer, the same blurred overlay appeared and I couldn't figure out how to get rid of it. Finally tried to move an application that was stuck behind it by alt+tabbing over to it and then pressing Win+right arrow to move it to a different screen. Once I did that, the blur disappeared.