I never knew the "L" key command! What it does (on MacOS) is it turns the Secondary Display background Black (tapping it once), instead of the default color, set in Preferences/Interface/Background (Medium Grey in my case). Tap it again and it gets rid of the menu bar and goes Full Screen (on the Secondary Display). If you set these background colors to Black, it does only the Full Screen mode.
Had no idea what you meant by "Lightroom Lights", you mean the background color on either side of the image. That's why it switches to the next desktop screen, because it only affects the Secondary Display, on that screen.