I have a laptop with two 4K external monitors (bigger than the laptop
screen). Just after boot, only the laptop screen is active, and
everything is OK (well, except that I would prefer an external monitor
to be detected automatically, but that's another issue).

After logging out, the laptop screen and an external monitor are both
active (the driver[*] cannot handle 3 screens, thus the other external
monitor is off, which is OK).

So, basically, the configuration is 2 screens of different widths,
which matters in particular for the menu bar. By default, the width
of the 4K monitor is taken into account. But when I use a menu, the
display temporarily switches to the width of the laptop screen. This
doesn't prevent me from using the menu, but it is a bit annoying and
makes the menu a bit less easy to use.

[*] This is the proprietary Nvidia driver, as the nouveau driver
does not work with more than one screen.