Based on 1.11.2 decompiled using MCP 9.35 rc1
Mob spawners do not test if the entity to spawn collides after calling net.minecraft.entity.EntityLiving.onInitialSpawn(DifficultyInstance, IEntityLivingData) (respectively the methods overriding it). This could result in mobs suffocating if the method onInitialSpawn changed their position.
Note: For naturally spawned mobs this is tested.
Currently blocked by MC-111747
- Setup a repeating command block and a chain command block behind it
- Encase a chicken in a 1 high space near the command blocks
- Use the following command to place a mob spawner
→ At some point a baby zombie starts riding the chicken even if there is not enough space above causing the zombie to suffocate