If you are working in a multi-room environment and you open the 3D navigation window, you will see that one of your rooms will be outlined in blue, identifying it as the room that is currently active within your project. The active room is also identified in the layout window by its walls, which will be a darker grey color as compared to the others.
The blue lines in 3D navigation are only visible when you are in the Coverings or Panels tabs of the Components Window.
To deactivate these blue lines, press CTRL+F9 on your keyboard.
This article is valid for the 2017 version and later.