Spiders spawn in an enclosed area where no 2x2 square of opaque blocks exist. See attached.
I made this enclosed area to catch spawned mobs. Rows of trapdoors are placed in lines in the floor. I placed lower-half slabs to prevent spawning on every second slab. I would expect spiders to not be able to spawn in there.
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The picture shows lots of 2 x 1 x 2 places where spiders can spawn.
Spiders need a 2 x 1 x 2 open space to spawn in, but only one solid-top block to spawn on. In the screenshot, they can spawn on the solid blocks between the slabs and trapdoors, with the space above the trapdoors being part of the 2 x 1 x 2 open space.
@GoldenHelmet I wish you would specify the dimensions. I don't know what you mean by 2x1x2. How many blocks vertical, and how many blocks horizontal can spiders spawn?
The reason I am commenting, I have spiders spawning in masses on a one-block space. These people creating bug tickets aren't crazy, and they shouldn't all be marked as resolved!
@Rocks I'm pretty sure they mean X x Y x Z, so a 2x2 space that is 1 block tall. Even though the wiki and some creeper farms seem to be under the impression that they spawn in a 3x1x3 space. It looks like we're gonna have to use the alternating button style of farms, instead of the spawning platforms.
Possibly linked to issue MCPE-56833. iron golems spawning on half-slabs.
In both issues it seems mobs ignore half-slabs when checkng spawn conditions.