When four stairs are placed as in the attached image and two diagonal ones are waterlogged the water doesn't behave as expected.
Expected behavior: no matter the outside conditions the two dry stairs should get waterlogged too (water source block propagation).
Actual behavior: The other stairs stay dry if there's eg. air under the stairs.
It only works as expected when the stairs are on solid blocks.
Most likely related to MC-228434 and MC-127030
The water fluid logic with waterlogged stairs apparently does not ignore the blocked blocks (two with normal and three with corner stairs) completely.
The water can't and wont flow there but the fluid checks still seem to check if it could flow there as if it were a normal water source block.
Maybe the order of checks is wrong? As in the water source block logic first checks where it will flow as if it were normal and then where the stairs block it's flow. Reverse that order and it should work properly I assume.
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@Felix Huang - how did you get to that conclusion?
I'm pretty sure it's not lag or a server issue because the bug is predictable repeatable. Did you even try to replicate it? (Why post your assumption without any testing?)
I noticed it on my server (spigot), replicated it there and then tested different orientations in creative mode locally without any server (and no lag I'm sure) and to make that screenshot.
It's even dynamically testable by placing blocks under the floating stairs after two were already waterlogged.
Depending on the placed blocks it suddenly behaves as it's supposed to (first only on dry stair gets wet, then the other).
And how is lag supposed to lead to the screenshot I attached? Exactly the same lag at exactly the right moments when I placed water but only in two of the four different examples?
This is probably the same core issue as MC-212829 – water in waterlogged blocks doesn't spread / create new water sources if there's air below.
I'm pretty sure this is related to lag or server issues because if you or a server is lagging, water blocks don't spread.