I’ve been using XMonad for a few years now (lately in combination with GNOME) but just recently I have found a way to make it work in a useful way in the context of a presentation with a projector as a second screen.

In general, XMonad does have decent multi-screen support. Building on Xinerama, the default configuration provides key-binding support for three screens; how workspaces are mapped to the screens and how this mapping can change, respectively, can be pretty confusing at first, though – it is best explained in here. Whenever I want to use a projector things get pretty tricky, however.

The thing is: Using a projector as a second screen, I want to have exactly the same workspace (accordingly the same windows, same layout, obviously) on both screens. Apparently, I could do that by ‘mirroring’ the screens and thus circumventing XMonad multi-screen support. I’m not too happy with it, though, as, for some reason, I can’t get xrandr make show a reasonable resolution on both screens in that scenario – part of what is shown on own screen is hidden on the other. Disabling mirroring, however, it is impossible for XMonad to show exactly the same on both screens – obviously, this is a restriction imposed by Xinerama as it manages all workspaces across screens as one large workspace under the hood.

The cure for that disease is the trick I found in another reddit thread: same workspaces on multiple monitors. So let’s assume the following setup: Workspace 1 is shown on your projector screen, Workspace 2 is shown on your internal screen. You start a VNC server on workspace 1 on localhost:

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x11vnc -usepw -clip xinerama0 -noxdamage -geometry 1920x1080

(You might need to replace ‘xinerama0’ by ‘xinerama1’. ‘1920x1080’ conforms to the resolution of my internal screen. ‘-noxdamage’ is optional but seems to be recommended.)

On Workspace 2 you start a VNC client:

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vncviewer ViewOnly=1 UseLocalCursor=0 localhost:0

(Again you might need to replace ‘localhost:0’ by ‘localhost:1’.)

Well, now you get to see your projected presentation on your internal screen, too, showing the X display of your VNC server screen! Move to Workspace 1 and start your presentation (or your live coding, for example) while you actually watch your presentation on your internal screen on Workspace 2! As a bonus, during your presentation you could easily do things which are not intended to be shown by the projector and/or move windows from an internal workspace to Workspace 1 when they need to be shown.

Starting the vncviewer with the ‘ViewOnly’ option makes it necessary that you actually make your live in-presentation changes on Workspace 1, as I said (while watching your own changes on Workspace 2 on your internal screen). That’s my recommendation anyway, in my experience the viewer setup thus works more stable and some keyboard inputs wouldn’t be translated correctly from the viewer to the server using a non-ViewOnly mode.