Affects Version/s: 1.15.2, 20w49a
Fix Version/s: None
Spiders can currently spawn in one block high spaces if the top block in transparent. In more detail, the current spawn code requires there to be a space of 3x3x1.5, however, the extra half block required gets ignored if the top block is transparent (glass for example).
I was trying to create a spawning platform limited to spiders, and placed a platform of opaque blocks, and two blocks above that I placed glass blocks. I expected this to still allow spiders to spawn, but they did not. I then changed the glass blocks to another transparent block, bottom slabs, and spiders subsequently spawned.
Steps to reproduce:
Open a new world.
Create a platform with opaque blocks.
Leave a one block high air gap.
Create a platform with glass blocks above the air gap.