Monsters spawn at daytime at y=256 and don't burn

Why not disable spawning altogether at level 256?

Confirmed for 15w46a.

The fix to MC-81067 means the mobs no longer turn black but this is still an issue (and the mobs also don't burn) in 1.9 Pre-release 4.

Please link to this comment in the description

The following is based on a decompiled version of Minecraft 1.9 using MCP 9.24 beta.

The reason for this is that the method net.minecraft.world.World.getLight(BlockPos, boolean) uses for y values higher than 255 the light level of the block at the height 255. If there is a block at this position (like it is shown in the screenshots) the light level is 0. Instead the method should return 15 as no block can be above this position to prevent the light from lighting this position.

public int getLight(BlockPos pos, boolean checkNeighbors)
{
    if (pos.getX() >= -30000000 && pos.getZ() >= -30000000 && pos.getX() < 30000000 && pos.getZ() < 30000000)
    {
        if (checkNeighbors && this.getBlockState(pos).useNeighborBrightness())
        {
            int i1 = this.getLight(pos.up(), false);
            
            // ...

            return i1;
        }
        else if (pos.getY() < 0)
        {
            return 0;
        }
        else
        {
            if (pos.getY() >= 256)
            {
                // Replaced this
                //pos = new BlockPos(pos.getX(), 255, pos.getZ());
                return 15;
            }

            Chunk chunk = this.getChunkFromBlockCoords(pos);
            return chunk.getLightSubtracted(pos, this.skylightSubtracted);
        }
    }
    else
    {
        return 15;
    }
}

Code analysis by Marcono1234 is good.
I confirm that the light is calculated on the block at 255 :
mobs burn on transparents blocks (glowstone, piston, barrier).
mobs spawn when it is the day if they are on a solid block (stone).

Please do not mark unreleased versions as affected.
You don't have access to them yet.

confirmed for 1.12.1

not fixed in 1.13 and 18w31a, please reopen.

Can confirm that it has not been fixed in 1.13 or 18w31a.

Confirmed for 1.13.1.

Is this still an issue as of Minecraft 18w43a/b?

Still seems to be an issue, both for spawning and catching on fire.

Confirmed for 1.13.2

@unknown, no need to confirm this for older versions, this has been fixed since 18w49a.

Affects 1.14 prerelease 3.

Affects 1.14 prerelease 4.

Affects 1.14 prerelease 5

I can confirm that mobs don't get set on fire at world height as of 1.14; however I wasn't able to confirm yet whether mobs can actually spawn there in the daytime.

It is easy to confirm.

Create superflat world in creative mode.
/tp 0 257 0
/fill 0 255 0 128 255 128 stone