I have filed this report before. Probably, you are trying to tell me that the problem has been reported 1000 times and that it has been fixed. Maybe the first part of the sentence is true, but not the second. The problem persists.

At the risk of being redundant:

* After connecting and then disconnecting external monitors, the panel disappears, and there is no evident way of getting it back to my primary display (see below). If I use a graphical user interface, I would like to have an accessible menu entry that allows me to create a new panel. I do not want to waste my time fumbling around with scripts in the entrails of the window manager.

* Primary display: I would definitely ask you to provide a definition of what you call a primary display. For me, a reasonable approximation would be to say that if a laptop is disconnected from all external monitors, then the primary display is the only display that is left, namely the built-in display of the laptop. I am sorry to state such banalities, but the way things work in Plasma, there does not seem to be a consensus.

* The fundamental problem is that you as Plasma developers severely overestimate your capacity of offering "smart" solutions. The consequence is that elementary functionality fails on a regular basis. Just give your users the chance to reset  the system to a state that does not make it entirely unusable. 

Martin Strecker