This is a fork of Tom Clark's extension to make it work with newer versions of gnome-shell. The following paragraphs are his words.
GNOME Shell has a nice feature called "unredirection", which sometimes disables compositing for fullscreen windows. I think this is a heuristic to reduce latency in video games, but it makes a lot of non-video game applications (such as HTML5 video in Firefox) have vsync issues. This isn't really GNOME Shell's fault, since applications shouldn't rely on the compositor to implement vsync for them, but as a user who loves watching memes in fullscreen I don't really care.
Since I keep forgetting the command to turn this feature on and off, this extension adds an icon to the main panel to do so.