Comment 5 for bug 754451

That's interesting Kamus. Following what you showed, I did some further investigating. That "Leave fullscreen" button only seems to come up when you go to the "Now Playing" option on the left hand side. If you're full screen already from the library and click on the Now Playing option, the album art window takes up the main section of the screen. Bottom right shows a "Repeat Off" option, just like on my original screenshot, however "Fullscreen" does appear on top right. If you then double click the album art area to make it go full screen we get the "Fullscreen" button on the bottom right like you showed.

I'd still argue this is a bug/problem. That fullscreen option seems to only appear when you're in the Now Playing section of Banshee, which isn't the default (even double clicking an album to open it doesn't jump you in to Now Playing)