When reaching the other side of a nether portal the animation plays forever until stepped out of

Info for snapshot video creators

Here are two gifs which can be used to compare the two versions of the nether portal exit animation:

• Correct animation, used up until 1.2.5 (broke in 1.3.1 snapshot 12w18a) and is now used again as of 1.20-pre1: https://cdn.discordapp.com/attachments/399390463930400778/1097153888597069844/1point2point5-netherportal-anim.gif
• Incorrect animation, used from 12w34a to 23w18a and now fixed as of 1.20-pre1: https://cdn.discordapp.com/attachments/399390463930400778/1097156653687787692/1point19point4-netherportal-anim.gif

Note that the "Entering the Nether" message being absent in modern versions is a separate bug and is not yet fixed (MC-12789). The absence of inner faces in nether portal blocks is also a completely separate issue (MC-262512).

The bug

When you reach the other side of a nether portal the animation plays forever until you step out of it, even in creative. The animation is completely unnecessary here.
This bug first appeared in 1.4 snapshot 12w34a - versions 12w32a and earlier are completely unaffected (but many are affected by MC-217613).

I'd recommend fixing this and MC-217613 at the same time. A table explaining the expected behaviour can be found here: https://minecraft.fandom.com/wiki/Java_Edition_1.3.1/List_of_broken_features#Nether_portal_exit_animation

Refer to Ismael Rosillo's code analysis below for how these issues can be fixed (and why MC-217613 cannot be considered a duplicate of this issue).

How to reproduce

1. Build a nether portal
2. Enter it
3. Stay in the exit portal
4. The animation will start playing when it shouldn't