Affects Version/s: 1.15.2
Fix Version/s: None
Environment:Java version 1.15.2. Windows 10.
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.