Slime block diffuse skylight. This is different from 1.12.2 and would cause excessive lag when it moves.
Tests by adam279: Affects 17w50a, not the version before that.
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Make yourself a hut where the roof is made of slime blocks. You'll find with a diminished light level (not 0 but not 15 either).
Now build one of those world eater ( https://www.youtube.com/watch?v=FYc6IcVx0C8 for instance ) on top of an extreme hills biome and find your PC will burn before your world. Hint : it's not because of the TNT.

Some reason screen shot didnt go through. I reuploaded it.
Diffuse and defuse are two different words Mr. "Helper"
Then it's a good thing Ray updated the report to use the correct word 😉
Confirmed on 17w50a, 1 snapshot after MC-130858. Previous snapshots seem unaffected.

That's actually useful information, too bad we can't write it into the version field. I'll add it to the description.
Affects 18w44a.

Can you make a screenshot, please? This is resolved as fixed.
18w43c and before:
[media]18w44a:
[media]
Can you attach a few screenshots of this, please? I don't know what you mean with "defuse", I only know that word in "defusing bombs".