Webpage ping when active

I want a way for users to open this webpage, and whenever they are on that page, it updates the server that they are on the page. It should only work when the user is actually looking at the webpage (not inactive, like from switchings tabs). One way to do this which I have implemented is to keep pinging the server saying that I am alive.

This however causes a lot of load on the server and client side. I am using Google App Engine and webapp2, and was wondering if anyone knows a better way to do this.