This browser extension lets you control background playback of YouTube Music without switching tabs. That is all. Features:

• tries to build a new playlist when one hasn't been loaded yet
• browser action button (left click toggles playback, context click displays more options)
• keyboard-based playback control for play/pause, prev, next
  • how to customize keyboard shortcuts?
• open and pin YouTube Music if it is not open yet
• autoplay when &autoplay=true is found in URL (useful for bookmarks)

This extension is a hobby project, not affiliated with YouTube Music.