Awesome! The mail widget always displaying was actually an error on my part, and I've added your Spotify album art fix as well.
Just for the curious: keeping all widgets and buttons on-screen regardless of which app is frontmost has been an intentional design choice so far. In this sense, you can customize your current keyboard by running or quitting various apps, which ultimately means you can rely on the keys staying where they are once they appear and can form muscle memory for the key positions. Additionally, Back and Forward can be used with Finder and System Preferences and a host of other apps, which is why they are considered Finder controls by default and never hidden (I'm debating adding New Tab to this group as well considering it's really now a system-wide thing).
GoldenChaos-BTT 2.536-4 changelog:
- Unread mail widget no longer persistently displays when there is no unread mail
- Spotify widget now downloads album artwork only on song change instead of every second