The bug
Mobs cannot spawn with the top of a scaffolding block touching their hitbox. For example, a zombie cannot spawn if there would be a scaffolding block covering its eyes. However, the scaffolding would not block it's movement, so it should be able to spawn inside.
Since creepers and spiders are not tall enough, they spawn anyways.
This would also provide some interesting new possibilities for mob farms.
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That is incorrect. Mobs will spawn inside doors, with trapdoors or string inside their heads, etc.
The spawning algorithm requires a non-full block at foot and head height, as well as that the entity to be spawned will not collide with anything.
Further testing in a server I'm part of indicates this bug only occurs for 2-tall mobs - creepers, for example, can spawn with scaffolding in their head. This suggests to me that the mobs are (incorrectly) colliding with the top of the scaffolding.
Can confirm. And yes, spiders and creepers are able to spawn below/inside of the scaffolding, while zombies and skeletons are not.
Zombies are able to spawn on and inside of scaffolding while begin call by the gamerule spawn reinforcements.
Video: https://youtu.be/fnyA7a4v-6o?t=70
This is not only the case for scaffolding. The spawning rules specify that there may not be any block directly above the spot an entity spawns at. That is intentional.I'm dumb