Qt scaling and kwin on X11 is never going to work so it is disabled.
However, Qt uses multiple environment variables for subtly different
internal things. We don't set QT_SCALE_FACTOR in Plasma, using different ones.
An XFCE user was setting this, which kinda makes sense if your DE doesn't
have an appropriate UI.
I don't think it's relevant for main_wayland as wl_output.scale should
trump the environment variable.