When exiting swimming mode in a one-block high dry space, the player will remain in a swimming/crawling state, a hopefully intentional feature. This continues fine provided the block above the player is a solid non-transparent cube. However, if this is not the case, some closely-linked glitchy effects can be noted:
The player will appear to be standing up, instead of being in a crawling state. In third-person mode, the knees will be treated as the player's eyes.
The player will no longer be able to walk underneath solid non-transparent blocks (they can still move between transparent cubes by walking). It is possible to sprint back into them, but the player will still appear standing.
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discovered while testing
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At first I was gonna argue that this bug has nothing to do with 1.12.2 (new swimming is a 1.13 thing), but ever since the elytra's hitbox was changed about 2.5 years ago, the visually-standing-but-eyes-in-knees thing has been possible. Seems like blocks that have no repelling properties if you were to stand within them will switch you to 1x1 elytra mode the moment you swim under them like this. This bug used to happen with all solid blocks too, but 18w20a fixed it via MC-128472 (while overlooking the fact that it's still happening with non-pushing blocks like glass). I just wrote a wall of text over there that was partially about how it shouldn't be marked as fixed since it's still half-broken, but it was mainly elytra/animation talk.
But anyway... I guess that makes this a dupe of MC-128472 rather than a "relates to" like it currently is? Then again maybe not, since the other half of this report talks about the actual "air swimming" itself, which I also hope is here to stay, albeit with a proper animation so that it doesn't feel like some kind of bug...
P.S. It's incorrect to say that you can no longer pass under solid non-transparent blocks during the stand-up bug. Just need to sprint.
muzikbike
Fixed in 18w22a
muzikbike
Is happening again in 1.13-pre5, please reopen
muzikbike
Fixed again in 1.13-pre6
muzikbike
it's back in 18w30a
muzikbike
Definitely does not affect 1.13. Removing from Affects Versions.
muzikbike
Blocks MC-131116, not relates to
Makzevu
Replying to your comment on MC-133453: This can be repeated with an elytra in 1.9 easily. This was in the game for a long time.
qwerty23495
--> MC-96922
muzikbike
This bug appears to have gotten even worse at some point prior to 1.13.2-pre2, as the player stands up even below solid suffocatable blocks.
muzikbike
...actually, I'd consider this bug to be blocked by MC-90594 ever since some point prior to 1.13.2-pre2.
Makzevu
The fix implemented accurate crawling in the game, only toggleable by being under a block.