from http://javadoc.jenkins-ci.org/hudson/model/Action.html

If an action has a view named floatingBox.jelly, it will be displayed as a floating box on the top page of the target ModelObject. (For example, this is how the JUnit test result trend shows up in the project top page. See TestResultProjectAction.)

The Computer index.jelly page doesn't do this, but it should be trivial to add.