I am on a Dell 9360 on which I have installed elementary Juno. I have it attached to a Samsung 27 in monitor. I was using the two screens successfully unmirrored with the Dell on the left as the primary monitor and the Samsung on the right. I turned the Samsung monitor off for a minute and toggled the mirror displays setting. When I tried to switch things back, the interface indicated I had toggled off the mirror displays setting, but I got no dialog box asking if I wanted to keep the settings and they remain mirrored.
I shut the settings screen, reopened it, and now the mirror displays setting is toggled on. The Samsung is showing as the primary monitor.
I have tried detaching the HDMI cable (which is attached through one of these USB-C all in one hubs with an HDMI port, some USB 3.0 ports as well as an ethernet jack), rebooting, then plugging the HDMI again. Not sure what else to do.
Any ideas? Many thanks in advance.