The diving behavior of the Ender Dragon when perching onto the portal was broken by the fixes of MC-87942 and MC-91858. Previously, the Ender Dragon would dive down to the portal while spitting acid. Due to the AI issues caused by the fix of the aforementioned two bugs, the Ender Dragon does not correctly dive downwards when flying towards the portal, causing it to make repeated attempts to fly to the center, flopping back and forth while slowly descending.
Relates to MC-271337
Note that this is not a duplicate of MC-197201, that report is refering to the divebomb attack that was removed upon 1.9's rework of the Ender Dragon. This report is instead about the Ender Dragon's perching behavior that was broken in 19w08b.
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Jukitsu
The aforementioned bugs were purely visual/rendering bugs that despite being related to the Ender Dragon, and don't seem to have anything to do with dragon AI, so it's likely they were unrelated. I think that the bug might have been introduced by an internal rewrite of some sort.
SeaOfPixels
@Jukitsu the fix of those two bugs specifically caused a new bug to appear in successive versions, where the dragon's animation/hitbox/behavior were all broken. That animation issues from that second bug introduced in 19w08b were addressed at MC-144612, however the hitbox/behavior issues have been left in the game to this day, leading to MC-271336, MC-271337, and MC-158205.
Jukitsu
My early very surface-level testing seems to suggest that this may be due to an issue on how the Ender Dragon dives in general, it seems to struggle to get the proper vertical velocity when flying to a target, thus making unnecessary turns back and forth and perching too high
Jukitsu
@PotholedSea40 While it is true that this version caused a lot of buggy behavior from the dragon, I don't think it is related to the fix of the bug. My best guess is that in the process of fixing the bug they also overhauled other parts of dragon code, breaking its AI in the process.
Also, I think I've found the issue. It seems comically stupid, but the ender dragon seems to (roughly) get a tenth of the vertical velocity it supposed to have while flying. At least, that is the case compared to 1.12.
I'll provide the details a later date, when I figure out how to properly get the decompiled 1.13.2 code and after further testing.
Jukitsu
If it is confirmed that this is the issue causing it, I suggest we move to MC-201937 as it is more descriptive of the issue. There was no point splitting the issue from the first place.
SeaOfPixels
Jukitsu
I see, however if my hypothesis is true, which I'm more and more convinced it is, then it means that the 2 issue corresponds to one same bug. Maybe I should make another bug report which would be more descriptive of the issue (namely incorrect velocity) if I have enough evidence?
Jukitsu
I made a new bug report further detailing the root issue that causes both bugs here: MC-272431
Can confirm for 24w21b
can confirm that this is as I have seen it for years but never realized it was a bug