@sujikawe As far as I understand your concern, it‘s because Spotify in the sidebar is set to work floating. It’s meant to behave like this.
You can set it to „fixed“ by clicking on the pin 📌 on top of the sidebar, though. That would make Spotiy in the sidebar open next to your main window just like a split screen. And it will only close when you deliberately do that.
You can of course resize the width of the sidebar still.
