This week the bug-fixing spree of the past few weeks wound down as feature work and user interface polishing moved into the foreground. So let’s start out with something pretty darn user-visible:
The outline itself is not a new feature. It won’t overlap with the window. It might closely match the colors of the window, in which case your window will look a bit thicker, but it’s not 100% opaque. It probably won’t 100% match the colors of your window, unless the background under the window has just the right color.