OK, so there are a few steps to fixing this bug:
1. sudo apt install gnome-tweak-tool
2. Gnome Tweak Tool > Windows > Titlebar Actions > Middle-Click = Toggle
Maximize Vertically
3. Log in using Xorg ("Ubuntu") for it to work (not "Ubuntu on Wayland").
Step #3 sounds like an obvious bug. But #2
It appears all middle-click titlebar actions don't work when in a
Wayland session.
** Summary changed:
- Vertically maximize on middle button click does not work in Wayland sessions
+ Middle-click titlebar actions don't work in Wayland sessions
** Description changed:
- Add support for
