It is possible for the player to end up intersecting several blocks that, while not full cubes, still have collision and prevent player movement. Unlike with full blocks, however, these blocks do not attempt to push the player out of them, and instead just keep the player within without opposing any of their movement until they exit again. This behaviour of players being within the block is undesirable due to it causing several glitchy effects, such as the third person camera being pushed inside the player and (as of 1.14) the inability to sneak. Having these be fixed could also make truly fixing MC-166718 possible without side effects.

Affected blocks:
Doors, trapdoors and fence gates (when closed on the player)
Several other blocks such as slabs, stairs, soul sand and fences, if pushed into the player via piston