Caffeine didn't work for me in Ubuntu MATE 14.04 (Trusty Tahr), I could not be bothered with patching it so I implemented it myself. Also works on Ubuntu MATE 16.04 (Xenial Xerus), Ubuntu MATE 18.04 (Bionic Beaver) and Ubuntu MATE 20.04 (Focal Fossa).

This is a Python application indicator which does exactly what it needs to do: it provides a notification icon that allows you to inhibit the screensaver.

It works by calling mate-screensaver-command -i in a subprocess which inhibits the screensaver. An interrupt signal is sent to the process when Caffeine is disabled. It also disables dpms through xset.

It depends on the python-appindicator package, so that needs to be installed via apt-get install python-appindicator. In Ubuntu Mate 18.04 (Bionic Beaver) the coffee cup icon requires installing the faenza-icon-theme package (apt-get install faenza-icon-theme). Use the bionic branch for a version that's completely compatible with Bionic.

The Python module appindicator has been deprecated in favour of; gir1.2-appindicator3-0.1. The coffee cup symbol also still requires the faenza-icon-theme to be installed.