When Choosing a Backend Fails

There was a recent GDK release into zesty that now probes for Mir over X11. This can cause issues when still using an X11 desktop such as Unity7 when a Mir server is running at the same time.

A common way to test Mir is to run it on top of X, which is called Mir-on-X. This means there are now two display servers running at the same time.

An example of an issue this can cause is gnome-terminal-server. It will attempt to spawn its clients on Mir instead of X11 once the Mir server is opened. You now attempt to spawn a new terminal which causes the gnome-terminal-server to crash since it now tries to spawn on Mir but you already spawned terminals on X. As you can imagine this is frustrating to your workflow!

A simple workaround is to add this to your ~/.profile:

if [ "$XDG_CURRENT_DESKTOP" = "Unity:Unity7" ]; then
    dbus-update-activation-environment --systemd GDK_BACKEND=x11
fi

Depending on your desktop the “Unity:Unity7” bit will change.

As more toolkits will start to pick other display servers as their first pick more of these issues will become possible. Other environment variables to consider:

SDL_VIDEODRIVER
QT_QPA_PLATFORM

A bit more detail on the issue can be found here:

Choosing a Backend

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