So,fast check. The Qt version works.

The gPodder Bug is a little bit different. The current track continues to playback while the new is added to the playlist.
At the GTK version the playback of the current track stops and starts from the beginnung again while the new file is added to the playlist.

I would say the playback of the new track should start immediately and perhaps we should add a bookmark on the last position of the old track. If the user press "playback" on gPodder he expects a prompt reaction regarding this clear command.

Furthermore the playlist is not a stacked, but a seperate window. Perhaps you should open it modal? With the main window as parent? (I'm not sure)

And the menu looks a little bit crowded.
Hard to serve on the small screen. But works so far... except the translations.

Perhaps it is a little bit cpu hungry. But perhaps I have at the moment simpy to much applications open. Will report if this problem persists.

Edit:
Ah, it's the git clone from sunday morning. Not from today.