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