Install xfwm4 by entering the following commands in the terminal:
sudo apt update sudo apt install xfwm4
Description:
window manager of the Xfce project
This package contains xfwm4 which is the window manager for Xfce4. It is designed to run with the rest of Xfce4 but makes a capable window manager for GNOME or KDE. . Install xfwm4 if you want a small, resource saving window manager which complies to the standards defined at http://www.freedesktop.org/. Without xfce4 installed this only gives you a window manager not the full environment.
Homepage: http://www.xfce.org/
Version: 4.12.4-0ubuntu1
Section: universe/x11