When sneaking on top of a shulker or boat, the player cannot move in any direction. Sneaking from a block onto these entities will cause the player to halt instantly, then occasionally snap forward during vertical movement (e.g. the shulker peeking). The player must stop sneaking in order to move.

How to reproduce

1. Spawn a shulker
2. Place a block next to it
3. Stand on the block, begin sneaking, and walk forward
   →  Progress halts immediately after the shulker becomes your primary support block

Expected behavior

Player should be able to move at normal sneaking speed without falling off the side of the entity, as typical for sneaking.