I have Ubuntu Mate on my desktop at work and I've noticed some issues after adding in a second monitor.
At the login screen, my display is mirrored across both monitors.
On my lock screen, only the primary monitor is displayed, the other stays black till I log in. Also, because I have a 3840x1080 wallpaper set to span, the image that is displayed on the lock screen is scaled down so that the x dimension matches the 1920 x dimension of the screen. This leaves the top and bottom of the screen with a weird green gradient.I'm hoping that by allowing the second monitor to be visible when at the lock screen, the image will render properly, across both screens. If I can't display both screens when locked, how can I change the image that is displayed for the background so it is independent of the wallpaper for the logged in user?