Another tricky feature, rounded corners! Took several days, but I believe I've got a pretty complete implementation.
You (manually) set the window corner radius and whether to force-clip the window. You can set radius per-corner to match GTK 3 apps. It works correctly with subsurfaces, blocked-out windows, transparency, gradient borders, resize and other animations.
Optimization-wise, opaque regions and even overlay plane unredirection work where possible!
Also, we've now got a little #niri setup showcase thread :blobcat: https://github.com/YaLTeR/niri/discussions/325