A bit of progress - from this post:
https://forum.manjaro.org/t/do-nothing-on-laptop-lid-closed/35550
I added the line:
IgnoreLid=true in /etc/UPower/UPower.conf
I already had the change(s) in /etc/systemd/logind.conf
So now, the external monitor does stay on when the laptop lid is closed. Both while booting (and getting to the login greeter) and once logged-in.
However, all windows are confined to the size of the laptop screen, rather than the larger (in resolution) external monitor.
When dragging a window, the mouse continues to move, and the workspace will wrap at the screen edges - but the window is blocked by the laptop screen size, and does not follow the mouse to the edge of the screen.
So I can’t place any windows towards the right or bottom of the display on the external monitor.
I do have the external monitor set as the primary monitor.