This is something that bugged me in the past - ideally the devs could write a new selector for stream signals with a user-definable fade-out/fade-in time when switching between signals
I had a look into this, using a code component after the selector you can cause the new signal to fade in when the selector changes. That's pretty easy, the hard part is fading the old signal out. By definition when the signal path changes in the selector you no longer have access to the old signal so there's no simple way to fade it out! You need AFAIK both the old and new signals running at the same time in order to fade one out and the other in without clicks.
I've got an idea that uses two selectors both connected to the same streams. One selector routes the main stream, the other routes the previously chosen stream if that makes sense. Then you can fade between them easily. The downside is this requires two streams to be running continuously. Then again if you did it all in code you have all of the streams that would have been connected to the selector running so there's still the chance to reduce CPU if you have more than 2 streams connected to the selector!
Whew! Don't know if any of that makes sense, I'll try to build a schematic to do it - unless someone else has a more elegant solution