Cerb allows you to get a lot done from a single page using popups. For instance, when viewing a ticket worklist you can open cards for each row to quickly review them:

Instead of clicking on the tiny (X) icon in the top right to close the window, you can simply press the <ESC> key.

If you have several popups open and you want to close one of them, click anywhere on that popup and then press <ESC>.