There are still other issues though. The performance of this page feels pretty bad in general.
works in pretty much all browers excepting firefox which doesn't have CSS scroll animations yet, but the nightly version does, so it should be generally available soon.
demo site: https://pete.design/tw-fade
github: https://github.com/petekp/tw-fade
npmjs: https://www.npmjs.com/package/tw-fade
if you use it i'd love to hear how it goes and if you have any feedback.
I don't know if this page is a demonstration of your plugin, I'm guessing yes but I can't see any masking going on, it seems to scroll just like a normal page but much more laggy.
EDIT: Oh I see in your comment now, it doesn't work in Firefox. My mistake.
arrow keys also seem to work fine but you have to click-to-focus first.