Steps to reproduce
Load [mediaInline]
This world contains a 32x32 sand platform in a desert biome with chains of each orientation at y +2 from the sand. I.e., there is a full block of air between each sand and the chain above it.Watch the platform for a few minutes.
Switch to night, then watch the platform for a few minutes more.
Expected result
Rabbits spawn on the platform during the day, and spiders spawn on the platform at night.
Observed result
Nothing spawns.
Impact: I discovered this issue when trying to use chains to spawn-proof against camels in a desert rabbit farm. I am trying to design a farm for the next update, wherein camels will attempt to spawn in the desert. The chains need to be at Y +2 from the sand to block camels because because of MCPE-190700.
Differential diagnosis:
* This issue is not a result of chains cancelling all surface-cap spawns below them (like lower slabs do). If you move the chains up by one block so that there are 2 blocks of air below them, then rabbits, spiders, and all other mobs will spawn.
* This issue is not due to the chain material type. If you replace the chains with iron bars or cauldrons then rabbits and spiders will spawn.
* Since the issue is not related to the chain material property or the spawn spots being classified as underground vs. surface, it could be due to chains having a default collision height that extends slightly below their space.