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 😉
Related to MC-128952 - Slime Blocks non-transparent
Invalid. Light isnt Affected. It gets darker as intended the further away blocks and entities are.
Edit:I thought it meant light from a block lol. sorry
Of course light is affected... Look at the debug screen. If there's no reduction, skylight is 15, but clearly it's not.
Skylight is 15 normally, the slimeblock acts like a leaf-block now. Sky-light will now diffuse, which means it gets darker over distance as the screenshot clearly shows since the value is 9 and not 15. Which wasn't noted in the patch notes and has game breaking experiences in normal slimeblock contraptions used in flying machines, like the ones used in the supported realms mini-game "missile wars" by cubehamster and sethbling. Why is it gamebreaking, because the current light engine cannot support it.
I'm Dumb, this is confirmed. But at the same Time I think this Works as Intended, But we'll see
Also this isn't a duplicate of
as it is marked invalid.
UPDATE: It was a Bug
[media]@unknown you can use !attachment|width=100%!
to get rid of the horizontal scroll bar.
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".