It would be a very good thing to have passage indicators on the edges of the screen you're viewing when editing the active background. Ones that show, where the next room doesn't have any walls yet.

This enables the editor to place screen edges in the wanted places much more easily, as well as assist aligning the walls properly from one screen to the next. It could be as simple as using the green arrows on the tiles where the next screen doesn't have any active background at the very edge.