Turns out nowadays if I want my Screen session to not die when the graphical desktop environment crashes (or I log out of it), I have to start Screen from a VT and then reattach from the DE and do a lot of "export DISPLAY=:0.0" in the individual Screen terminals just like it's...some year a long time ago.
@johns Are these systemd-using machines? If so, does "loginctl enable-linger johns" help? Apologies if you already went down that rabbit hole.
@johns TIL about that, but indeed it seems to preserve tmux from killing my lightdm session here.