The SwiftUI/shader method was a bit of a dead-end since it would've meant I had to make a fake Mac window from scratch and then apply the shader. Which is silly and goofy and dumb.
But you know what's even goofier and dumber but more efficient? Abusing private APIs like CGSSetWindowWarp. Making good progress here.
The window remains fully interactive and mouse clicks get translated too, so you can do precise things like use a slider.
https://hachyderm.io/@samhenrigold/113275854041331969
oh also I hooked it up to a spring animation and made it fully interruptible weeeeeeeee