Same here. Also, the current beat highlight is always red and so it's invisible when the content is red as well (i.e. kick by default).
Beside this, very cool!
I'm getting a little popping on Firefox Windows, which disappears if I remove the kick step on second row of the default bosa nova. Nice work anyway, the UI is immediate and instant play.
Same, with Firefox on Linux (via Flatpak). The UI's great, though.