Is there a single command to simply disable the "out-of-focus" effect? In Dconf manager I only see different setups for this behavior but none to just disable it.
In Gnome Shell, there are three possible window focus modes, (from dconf-editor) 1) “click” means windows must be clicked in order to focus them, 2) “sloppy” means windows are focused when the mouse enters the window, and 3) “mouse” means windows are focused when the mouse enters the window and unfocused when the mouse leaves the window. Thus, to disable the focus follows mouse, you need to set it to "click":
gsettings set org.gnome.desktop.wm.preferences focus-mode 'click'
This is actually the default setting in Ubuntu and in many Gnome Shell installations. So also the command
gsettings reset org.gnome.desktop.wm.preferences focus-mode
would usually revert to the "click" setting.
This setting is also exposed in the utility Gnome Tweaks (Windows tab). It is not installed by default in Ubuntu.