The window opening animation is now live as part of niri v0.1.2: https://github.com/YaLTeR/niri/releases/tag/v0.1.2
I'm really looking forward to more animations, but wow they sure do need a lot of care to get right in all the edge cases.
Also, I added a way to programmatically invoke compositor actions, and turns out that's quite useful for making video demos!
GIF