This Fabric mod for Minecraft 1.21 enables the debug feature
renderServerSideHitbox which is present as an unused function in the
game's code.
Enable hitboxes with F3+B and this mod will show the server-side hitboxes
in green in addition to the normal client-side ones in white.
Usage examples include observing desync after sprint-punching entities or standing on a client-side piston head.
Available on Modrinth.