I seem to notice that when connecting an external Display to Ubuntu MATE, the priority set on which monitor is the primary monitor does not appear to be respected. It always seems dependent on location of the monitor on the Display settings area. Screenshot is attached.
I have a USB-C Thunderbolt capable Dock connected to a 25" Spectre Monitor with AMD Freesync ( Running at 1080p); My Laptop (ryzen 5 with built in Radeon Graphics) is using the Displayport over USB-C standard.
When I connect to the external display, all the program windows seem to go to the External display even though the Laptop Display is set as Primary. Also, the Welcome Screen always launches on the External Display no matter what display is set as primary.