I can't take a screenshot, but I can edit one to show kinda what it looks like:

Right on the edge of the BL window. It happens with everything. Firefox/Paint.NET/Folders, roblox studio, xfire, etc.
If I have firefox maximized, and BL in the center of the page, when I change to a tab with a different colored background, that rectangle on the tab will load .1 seconds faster than the border, so it looks like a rectangle was put in the background, for that fraction of a second, on the new tab.