Amazing site. I noticed one thing that could easily improve the feel of the site — the sheet music playhead which is outside of the canvas.
You are using fractional units to position it, but you're also using left/top/height properties which can't be fractional.
If you convert these to GPU translated layers via translate, you'll get a smoother playhead since these can be positioned fractionally, plus you'll cut down on the repaints!
It's a firefox bug. I needed to add a very fine rotation for it to render subpixel, so you can try again. You can also see the repaints if you open up the dev tools: https://i.imgur.com/A8btxpu.png
You are using fractional units to position it, but you're also using left/top/height properties which can't be fractional.
If you convert these to GPU translated layers via translate, you'll get a smoother playhead since these can be positioned fractionally, plus you'll cut down on the repaints!
Here's an example: https://codesandbox.io/s/magical-wind-mptqn?fontsize=14&hide...