There’s an interesting (to me) implementation detail in the A+B=-C lines on this page.
It's possible to show the true lines with the toy curve, but the X25519 animation at the top of the page is a cheat: it uses the correct slope IIRC, draws the dot following that slope for five wraparounds, but then cheats and jumps to the needed height and does only the last line to its destination.
If I'd done the actual number of wraparounds used in a typical X25519 point addition it'd be like TV snow, or would take hours!
It's possible to show the true lines with the toy curve, but the X25519 animation at the top of the page is a cheat: it uses the correct slope IIRC, draws the dot following that slope for five wraparounds, but then cheats and jumps to the needed height and does only the last line to its destination.
If I'd done the actual number of wraparounds used in a typical X25519 point addition it'd be like TV snow, or would take hours!