The GTK ThinLinc installer doesn't start on a fresh RHEL 10 system, text fallback is used instead. This is due to a missing dependency that gets installed directly when running the installer -> xorg-x11-xauth.
If I run "install-server" as root, the GTK installer starts without issue without xorg-x11-xauth installed. It looks like it is there is some check for the initial window that prompts the user to run sudo, that for some reason requires this package.
This isn't a regression in ThinLinc. I tried the same operation with 4.16.0 and got the same behavior as above. Note that I did this while being logged directly on the host as a non-root user, both when testing with 4.19.0rc1 and 4.16.0 I also tested this on a RHEL 9 machine. There, the installer doesn't fall back to text mode. RHEL 9 already has xorg-x11-xauth installed. Also note that in all cases above, Gnome has been installed via the "Server with GUI" dnf group.