Thanks for playing, glad you like it! Don't worry you're good, I want people to let me know about any problems they have so I can fix them and make the game better!
Although unfortunately that's a problem I'm not sure I can fix. That's a known issue that can happen when there's too much lag, which can happen on lower powered devices like Chromebooks. It's caused by the music and the midi player (which spawns the notes over the box, only becoming visible when they enter the box) going out of sync. There's a system to mitigate that, by slightly moving the spawn position of the notes up or down, which works great for when there's not too much lag. But too much lag overwhelms the system, so the spawn point is pushed down so much that the notes starts spawning inside the box.
I'm not sure how I could fix that without rewriting most of code for note spawning, which is obviously a pretty big project. I'll keep trying to see if there's some other way I can fix it, but for now, sorry it might just not work on that device