Using Fedora 22 I have a VGA cable to my monitor plugged into my laptop. I have configured the monitor to be primary screen. Unplugging the monitor it does not reset the laptop to be primary - this worked fine under Fedora 19 (for instance). Reproducible: Always Expected Results: I would expect it to retain knowledge of different monitor layouts and correctly switch between them (including retaining which is primary and redisplaying the task bar).
I am experiencing a similar problem. I will try to describe the problem clearly. I have a laptop with a external monitor through DP. Now I plug in the external monitor, and go the system setting and set the external monitor as the primary one and enable both monitor. Now if I remove the external monitor, I will get a desktop without widget in my laptop's screen. If I plug in the external monitor again, the plasma's widget in the main monitor shows correctly but the secondary monitor (laptop screen) is black (no wallpaper).
Good news. We have fixed a number of issues which lead to this bug. Would it be possible for you to test 5.7, or better yet git master (perhaps from KDE Neon or openSUSE tumbleweed)? I'm closing this bugreport, since we already have the same root-cause reported in others. If need be, it can be re-opened or a new one filed, and we take it up from there. Thanks for the patience!