Fade out with half transparency previous track listings from previously played stations
Some radio stations provide tracks listings information that Shortwave displays in the sidebar on the right, but many (most) stations don't, so when you switch stations the old tracks sit there forever, and they are visually misleading.
Short of clearing the widgets upon station change (which I suppose some users might find too extreme), my recommendation would be make these rows 50% transparent, since GTK allows doing that easily. It would make it clearer that they are old tracks, and would make them less visually prominent in the UI, too. It might help to have a label above them that says "The currently playing radio station does not provide track information." or something like that, but maybe that's not something you'd desired, hence the subtler transparency approach.
This transparency approach could be used even when switching to stations that do support track information, to mark the tracks from the previously played station, and eventually they get "pushed out" (pushed down) after a couple of new tracks from the new station have played, but at least there will be a distinction on where they were played, for a while.