Here is a possible implementation:
https://github.com/strawberrymusicplayer/strawberry/pull/906/files

See here and here.

Sorry my fault, I was under the impression that Strawberry retired the now playing widget in favor of the context tab in the sidebar.
I only realized just now that it's still available - if I enable it, I see the GUI option as well.

But since this option actually applies to the whole sidebar instead of just the widget, wouldn't it make sense to show this option under "Behavior" or "Appearance" in settings?

@mbnoimi This is actually a good idea. I wonder if the developer thought about this already.

@fatso Found it. Thanks for the info!

For anyone else looking for this, on Mint linux 20.3, the full path is:
/home/your_user_name/.config/strawberry/strawberry.conf
(.config is hidden, so use CTRL-H to see it)

@cherrystraw I can't speak to the program coding involved, but a new DB field (possibly in playlist_items ?) would get the ball rolling...although I'm certainly not telling Jonas anything he doesn't already know.

A bunch of Qt updates came through Pacman last night, a few for Qt6 too, and it seems like the icon appears now, so this is resolved 😄

See:
https://github.com/strawberrymusicplayer/strawberry/wiki/Import-collection-library-and-playlists-data-from-Clementine

Ahh found it. You can define the behavior in Tools > Settings > Behavior > Double click a song > Open in new playlist