The bug
Visually the height of chicken jockeys is 1.5 blocks, however, under careful observation their hitbox actually reaches closer to 1.5025 blocks, making some interactions with them inconsistent.
Observed behavior
When trying to trap chicken jokey in 1.5 block tall area you can’t place top slabs above their head.
Expected behavior
You would be able to place a top slab above chicken jockey's had.
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