Mobs not performing any collision checks.
not sure i need to explain this one too much, hopefully the image says enough.
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[Mojang] Mega_Spud (Jay)
gruva
Yes, this was tested for structures only.
In the structures they also spawned on the glass or open fence gates.
Auldrick
Confirmed in Windows 10 1.2.10.2. However, I can confirm it only for wither skeletons spawning in the Nether.
I also tested in the Overworld. With slabs (either bottom or top) above a 1-block gap, only spiders (and bats with top slabs) spawned. With slabs above a 2-block gap, endermen did not spawn.
migrated
endermen ignore water on top of a slab
migrated
I did some testing and noticed:
Glass (at head level) doesn't prevent mobs from spawning, but solid blocks do
Enderman can spawn at 1 or 2 blocks gap
Are mobs spawning in naturally this way?