when I am using an attach monitor, I noticed a behaviour which is annoying: even if primary display is set up to the attach monitor the windows stayed on the laptop monitor. This is the same when opening a new window which will always be open on the laptop monitor not on the primary display even if the mouse is on the primary and the launch made through a window or through the panel on the primary display. This is not a major issue obviously but it is annoying.
The primary monitor concept has no meaning for a window manager and does not influence where windows open.
It was worth it mentioning it to try to improve the user experience on Plasma. I am wondering how windows, macOS and Gnome are able to achieve that without the concept of primary monitor (or the panel in plasma).