Active window should be marked somehow so the user doesn't lose his place. Bear in mind that there are three areas that potentially might need highlighting: 1) the active window 2) the window that's focused via keyboard (arrow/tab keyboard navigation is currently missing but presumably it will be added later) 3) the window that the mouse hovers over, AFTER being moved. It isn't trivial to figure out the optimal UI here, so I recommend just copying whatever Gnome does for starters. Their overview is ten years old, so presumably its UI makes sense.