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Hostile mobs can spawn on top half iron, crimson, and warped trapdoors, but not on top half wooden trapdoors.
I have heard that mobs would not be able to spawn on top half trapdoors in the latest snapshots. I tested this in a superflat creative world at night and expected top half iron trapdoors to be spawnproof.
I used the fill command to create an area of top half iron trapdoors and moved away. Hostile mobs started spawning on the platform. I used another fill command to replace the platform with top half oak trapdoors and moved away to despawn the mobs. After moving closer no mobs were spawning on the wooden trapdoor platform.
The expected behaviour would be that mobs are able to spawn on either all types of top half trapdoors or no type of top half trapdoors.
- Create a superflat world with a spawnproof floor (such as glass) on easy difficulty or above.
- Move away from the spawn point and use the fill command to create a platform of top half wooden (oak, spruce, birch, jungle, acacia or dark oak) trapdoors.
- Set the time to night and move at least 24 blocks from the platform.
- No mobs will spawn on the platform.
- Use the fill command again to replace the platform with top half iron, warped or crimson trapdoors.
→ Hostile mobs will be able to spawn on the platform.