The problem#

Many applications have a fullscreen mode that has a different interface from their windowed mode. For example, many media players will show just the video in fullscreen mode but include media controls in windowed mode. But, especially if you have a large monitor, you may want to use that interface while only having the application take up part of your monitor.

The solution#

I could not find a solution that works on every window manager.

The window manager handles resizing the application when it switches to fullscreen, so the most straightforward way to accomplish this is to not run a window manager. Problem solved! Unfortunately, window managers are really useful, so outside of some niche cases where you're positioning windows with xdotool, that's probably not what you want.

There's a "fakefullscreen" option in some forks of the very configurable window manager dwm: base dwm with the fakefullscreen patch always does fullscreen that way, the instantWM fork has a hotkey Super+Shift+F that toggles fake fullscreen for a window, and awesome can be configured to do the same.

For more common window managers, there is a solution, but more than two virtual monitors requires an xorg-server newer than 21.1.10 (which is the most recent release at time of writing, so you would have to compile it yourself), and in my tests, it only worked on Compiz, and not Mutter, KWin, or Xfwm. Use xrandr 1.5+ to define virtual monitors on sections of your monitors and then maximizing or fullscreening applications should respect those boundaries:

xrandr --setmonitor lefthalf 960/217x1080/132+0+0 LVDS-1
# This is a hack, should be "LVDS-1", not "none".
xrandr --setmonitor righthalf 960/217x1080/132+960+0 none

where the the geometery specification format is w/mmwxh/mmh+x+y (mmw/h="millimeters width/height") and LVDS-1 is the name xrandr gives to my physical monitor. Note that xorg-server 21.1.10 and older have a limit of one virtual monitor per physical monitor which we can circumvent by putting the second virtual monitor on "none".

The details#

Splitting ultrawide monitors#

All of the discussion I could find about this online was about dealing with ultrawide monitors, where people wanted to make them act more like two or more monitors side-by-side. This StackExchange question links to much of the discussion including the dwm patch linked above and this Xorg change that merged December 17, 2023 (i.e. after the 21.1.10 release, the most recent at time of writing), which added support for multiple virtual monitors splitting up the same physical monitor. There's an open merge request in GTK to support that, so this may work better in the near future.

Separate from that discussion, I found that for KDE, the recommendation is to use Mudeer.

Behavior under other window managers#

There's a lot of window managers; I only tested the most common ones that were already installed on my machine.

In my testing, Xfwm (default for Xfce) showed the same behavior seen in that StackExchange question: it would choose one of the virtual monitors to display on and the rest of the screen would be black.

Mutter (default for GNOME 3) and KWin (default for KDE), both seemed to completely ignore the virtual monitors.

As stated above, Compiz (originally for GNOME 2, now default for Unity) does respect the virtual monitors and treats them like real monitors.

How to test#

As I mentioned, this feature isn't fully supported yet in existing releases of xorg-xserver, so I had to compile it myself. There's a page on how to build X from source with some helpful scripts that make it simple. One important note is to pay attention to the recommendation to compile only the modules you need; the relevant code change is in the module xserver. I did get an error about something "proto", so I added the proto/xorgproto and proto/xcbproto modules to the list to compile and it worked.

That page then explains how to run the Xorg binary, although I ended up just running it as root because I couldn't figure out how resolve the permission issues and decided it didn't matter for a quick test.

As a side note, I did try to test using Xephyr to just do this in a window, but as far as I could tell, Xephyr was completely ignoring the virtual monitors. I ended up using a laptop instead.