I've been trying out tiling window managers lately and I recently installed Regolith into my existing Ubuntu installation.
After logging out of my Regolith session and back into my Gnome session, I noticed the window titlebar buttons were missing. Gnome Tweaks only allowed me to get back the minimize and maximize buttons. To get the close button as well, I had to run the following command:
gsettings set org.gnome.desktop.wm.preferences button-layout ':minimize,maximize,close,'
Hope this helps someone else.