When leashing a happy ghast and jumping on top of them, the ghast rubbers back to the player. Repeatedly doing this can boost both entities to float or fly into the sky.
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This issue is caused by happy ghasts using the default parameters in the minecraft:leashable component. I was surprised when I saw this in the behavior file, because ghasts are 4x4x4 in size and default leash behavior has the leash start to pull the mob at 4 blocks away. Just glancing at the code I expected it would be annoying to work with leashing happy ghasts, but the fact it also allows this lowkey exploit is amusing. In addition to the unadjusted leash length, the anchor point for the happy ghast is at the bottom of its collision, so just leashing it, standing on top, and jumping makes the leash pull.
The bottom anchor is probably fine--it's intuitive given the ghast is just a big head with tentacles, and it will usually float above the player or fence post holding the leash. To fix this issue and make it impossible using a jump boost 2 potion you would need to make the "soft_distance" higher than 6.5. Although, allowing this behavior only with the jump boost effect is an interesting idea.
Since the upload is cut, here is the full video: https://youtu.be/4rGyEMVJMJE