Short version: LightDM doesn't quite load my i3 session correctly and fails splendidly at loading my vanilla Unity desktop session. I suspect it's a problem with LightDM's configuration or various startup files (e.g. .xinitrc, .profile, etc).
Now, when I hit the login manager and select i3 session as I log in, it shows a blank background. I have to load i3 by pressing Ctrl+Alt+F1 and running startx. When I do that, nm-applet no longer shows up in the lower right corner of my i3status bar as it did before. My bash shell also has a different font (it still highlights filenames, though). (Edit: htop is also suddenly not installed, which is really confusing.)
When I try to run a Unity session, I get a blank white text box taking up a small fraction of my screen that looks like it's supposed to be a terminal of some sort. I can't press Ctrl+Alt+F(1-6) and get back to a terminal, so I am forced to reboot. Fixing this problem isn't critical, but hopefully, it helps diagnose the problem.
I installed i3 a while back.
sudo apt install i3
Upon my first reboot, I was initially very happy with it. I tweaked some nautilus settings to not show the desktop icons so I could use nautilus.
gsettings set org.gnome.desktop.background show-desktop-icons false
After my second reboot, I logged in with my familiar Unity login manager and selected 'i3' as a session, I encountered a blank desktop background.
I followed the instructions here: to no avail, so I tried reinstalling ubuntu-desktop, resetting, and eventually removing my existing compiz configuration.
sudo apt-get install --reinstall ubuntu-desktop dconf reset -f /org/compiz/ rm -rf ~/.compiz-1 ~/.config/compiz-1
Here's my .xinitrc file, which I made at some point during this long process.
#!/bin/sh # # ~/.xinitrc # # Executed by startx (run your window manager from here) if [ -d /etc/X11/xinit/xinitrc.d ]; then for f in /etc/X11/xinit/xinitrc.d/*; do [ -x "$f" ] && . "$f" done unset f fi setxkbmap -option ctrl:nocaps # set PATH so it includes user's private bin directories PATH="$HOME/bin:$HOME/.local/bin:$PATH" VIMRUNTIME=/usr/bin EDITOR=/usr/bin/vim # alias to allow quick vga connection alias connect-vga='xrandr --output LVDS2 --auto --output VGA2 --preferred --right-of LVDS2' alias disconnect-vga='xrandr --output LVDS2 --auto --output VGA2 --off' # set primary display so that nm-applet icon shows up? untested #xrandr --output LVDS1 --primary # exec enlightenment_start exec i3
P.S.: I also got a 4K monitor just about when all this happened. In fact, this started when I had some issues setting the resolution to something that would work with my 8-year-old Thinkpad T420. It seems to work fine now, though.