When I created a mod to add ladders of all the vanilla wood variants, I discovered rapidly, to my dismay, that vanilla Minecraft has a very stupid thing: The code which allows an entity to climb through an open trapdoor as if it were a ladder, providing the block below is also a ladder, is hardcoded to the vanilla ladder.
This made no sense to me, and was also easily fixed with a small mixin (after I successfully made trapdoors crash the game briefly). Since then, I have abandoned that variant mod, preferring those others have done... but they don't necessarily include their own code for ladders and trapdoors!
So I'm releasing this. It's literally just the mixin. You can plop it into your mod folder and poof, any ladder that is an instance of the vanila ladder block class should work with any trapdoor that's an instance of trapdoor. Unless a mod did something really weird, this should work globally.
If a mod already has this, it may or may not still work with it but mine is likely not needed if someone else did it. You only need this if you get to the top of your ladder and can't climb through the trapdoor itself.
License is ARR. Feel free to use it in modpacks, and it'd be absolutely stupid to say not to use it in videos... But if you see it posted elsewhere, assume the unscrupulous individual who did so may have also altered it dangerously! If they're willing to go that far... shrug