Why this is an issue: This causes unnecessary lag to the light thread, particularly when doing stuff in void worlds.
From y=-1 to y=-2048 light level for skylight, as shown in debug menu is considered, but for below -2048, it always shows as level 15.
Block light is also considered, but this does not bear a large impact in relation, as skylight has to propagate all the way down.
How to reproduce the lag: get a void world. Fill in something like /fill -90 1 90 90 1 -90 stone . note lag occurs. Then, do /fill -90 0 90 90 0 -90 stone , then fill /fill -90 1 90 90 1 -90 air and then /fill -90 1 90 90 1 -90 stone . Note lag does not occur
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This is invalid. Light updates are only calculated until y=-16. Blocks below that simply inherit the sky-light values from y=-16.
The reason that the behavior changes below y=-2048 is that the queried BlockPos is converted to a long internally, decreasing the range of valid values. Hence, below y=-2048 the queried value wraps around and internally queries some large y, which is correctly returned as 15.
The lag you are experiencing in that case is likely MC-123584.
Do you have any proof that this is the case? Because in reality, no player will ever fall that deep without dying.