Previously, mobs did not spawn if there were no air blocks above the solid block chosen as the initial part of the spawn algorithm.
As of 14w25b snapshot, this is no longer the case. There is no obvious mention of such a change in the notes, hence filing this as a bug.
Creating a flat world with rail as the top surface results in no mobs spawning pre 14w25. From 14w25 and onwards, mobs will spawn despite the lack of completely clear spaces previously required to "seed" spawning.
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is duplicated by 5
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Comments 3
This probably happend with the introduction of the Guardian. Hostile mobs now must be able to spawn inside water. So maybe the check for an Air-Block was simply removed and only the collision with objects remains.
if so, than it's sadly not a bug.
In 1.7, mobs could not spawn on a superflat world that had tallgrass (or any non-solid non-air block at the top). In 1.8 this changed, and they could.
The fix here in 15w46a appears to be that now 'rail' (and variants like golden_rail) do not allow mobs to spawn, but other non-solid blocks (like tallgrass and snow_layer) do. This is based on simple experiments in 15w46a with superflat worlds of varying top-layer blocks.
Further experiments show that on a world with a golden_rail top, a small patch of air will cause mobs to spawn only on the blocks without rail. So it's not just the air-seeding, it seems mobs cannot spawn naturally on rail (even rail with nearby air). Mobs from spawners still can spawn on rail.
Jens Bergensten
Confirmed for 1.8 (due to MC-71518 and MC-71008).