Manjaro being an offshoot of Arch, they have plenty of guides and references, with How to Set up X11VNC Server being most useful at the moment. This box needs only a VNC server and apparently works with
‑xdamage for faster updates.
With the laptop plugged into an external display and Manjaro set up to use both displays, the X11VNC server feeds both to the client with the proper positioning, producing a truly panoramic, albeit scaled, view:
TightVNC on Windows does much the same thing, although (AFAICT) Windows doesn’t allow different background pictures on the two screens; that’s irrelevant to my mmmm use case.