With Tab you can show or hide the scoreboard on the left side of your screen. I would find it really cool if I could set the scoreboard to "Friend only" so only the scores of my friends would be shown. For me at least it would be awesome cause I play osu primarily with my friends and we are all rivals 
Edit: Tab to friend only.
Normal: Score board shows,
After first time Tab: Scoreboard goes to Friends only
After second time Tab: Scoreboard Hides
After third time tab: Comes back to normal
Edit: Tab to friend only.
Normal: Score board shows,
After first time Tab: Scoreboard goes to Friends only
After second time Tab: Scoreboard Hides
After third time tab: Comes back to normal