By clicking or clicking and dragging, Listener will highlight messages. This can be done in the Snooper too (hold shift if the mouse is disabled). This is a simple feature to be used as however you see fit. Marked messages will show up with a yellow tab (if tabs are enabled).
Marking messages as highlighted will also mark them as "read", and the readmark (the red line) will follow whichever messages you mark. The readmark is also cleared whenever you type something into a particular window with it. You can also clear the readmark with a key-binding (located in the Key Bindings options).