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@unknown's comment
The bug
In 1.2.5 SSP and before the little wobble to the whole viewport that occurs when you take damage used to indicate the direction the damage came from. As part of the 1.3 client/server merge, this behavior was lost.
Steps to view correct behavior
Revert to 1.2.5 SSP and start a new creative world
Build the contraption as shown in the image attachment below
Fill the dispensers with arrows.
Exit MC and use NBTExplorer to change the world to survival mode (alternatively play survival until you obtain those items or open a copy of a current world)
Facing forward, strafe left into the left pressure plate. Arrow will hit you and the viewport will tilt to the right, simulating the head whiplash.
Facing forward, strafe right into the right pressure plate. Arrow will hit you and the viewport will tilt to the left, simulating the head whiplash.
Facing forward, walk into the front pressure plate. Arrow will hit you and the viewport will tilt DOWN, simulating being hit in the top of the head by something
Facing forward, back into the rear pressure plate, Arrow will hit you and the viewport will tilt UP, simulating being hit in the back of the head and having forward whiplash.
This seemingly "cool aesthetic" actually has real gameplay value because it indicates to fighters where incoming damage is originating from.
Repeat all steps in latest version (no need to use NBT editors just open to lan with cheat mode to switch gamemode). Notice how the viewport will always tilt right no matter what.
Code analysis (MCP Names)
EntityLivingBase.attackedAtYaw
is not synced to the client, causing an animation present in the client to not be correctly rendered.
Related issues
is duplicated by
relates to
Attachments
Comments


Confirmed.
Confirmed all the way up to 14w18a
Confirmed all the way up to 14w29b
Confirmed to 1.8-pre3. please fix!
Confirmed tup to 15w46a. Please fix.

This would be great to have back, yes!
Confirmed to latest snapshot (November 18).
This would really really be a good (re)addition to combat. Please fix it!
It's just syncing one more field back to the client.
Thanks
Confirmed up to 16w32b

Confirmed for 17w16b

Confirmed for 17w17a
Confirmed for 1.12
Confirmed for 1.13.x and 1.14

Affects 1.16.1

Affects 20w27a
Confirmed for 20w28a
Confirmed in 20w29a.

Did the direction dead mobs fall ever depend on the direction of incoming damage?
Confirmed in 1.16.2
So sad to see "won't fix".
I know you don't have to do this, but do you have a reason for marking this "Won't Fix?" I'm more than familiar with some bugs just not being worth fixing, but when the problem and solution has been pinpointed by forge developers in the video here, I don't think it can fall in that category.
Please fix this! It would be immensely helpful! Why "wont fix"? Even if you won't fix it, just give a reason!

The issue tracker is not a discussion forum. If you disagree with an issue resolution, please bring it up on the MOJIRA subreddit or discord.

Why it can't be a toggleable option in Accessibility options ;-:
If you REALLY want it back, install this mod.
Look. I never knew that screen tilt was ever a thing so I never actually saw that as a bug, just a feature. I wouldn't actually think that this would be a bug in some way before I read about it and saw this video: https://youtu.be/pAJPc71YOnY
For PVP community this would be a good feature and without having to install mods in order to fix the bug, it would be great to have that in game. I think that servers that do not run modded version running this mod will still work the same, even if you have the mod installed, but I don't know if it actually works just by sending that piece of information to the client or it actually uses some clever math tricks to compute that kind of missing information in case that information is missing.
If the later is true, then using this mod may actually result in ban on some PVP competitive servers (Factions, UHC, Skywars, etc.) because this isn't just a visual effect like resource pack lowering fire and shortening swords (which is purely aesthetics to better see around you) which everybody can install and if someone doesn't have them - well that's their fault. This is actually real gameplay feature and it can give serious piece of information about whereabouts of other players attacking you. Kind of like minimap mod showing you whereabouts of other players in your proximity. There are players that for some reason cannot install mods, or don't want to. If you fix that bug and maybe add a little toggle option to turn it off (return back to the now classic left-tilt feature - actually bug), those who opt it out and turn it off won't be entitled to accuse players using that feature for cheating, because it will be vanilla game feature now. Of course, x-ray is something that doesn't exist in vanilla game, or at least nothing intentional. In previous versions, you could x-ray caves by putting your head into a glowstone or (in case of fast graphics) in leaves, though you wouldn't see unexposed ores anyway, just where the caves are, this is now considered bug (that has been fixed anyway) and many servers prohibit abusing bugs for your advantage.
However, screen tilting towards the inflicted damage is something that should be in game and as it turns out it has been before, it didn't work on servers though as the code for sending that bit of information to the client was missing. Seriously, how hard is to add I guess one little line of code to send that information to client amongst other ones?

I'll repeat myself: The issue tracker is not a discussion forum. If you disagree with an issue resolution, please bring it up on the MOJIRA subreddit or discord.

The comment section in the bug tracker is only meant for providing additional information regarding a bug report, it's not meant to be used as a forum for discussion. Please see @unknown previous comment on where you can hold discussions.
If you don't respect the rules of the bug tracker, you may be banned.

Confirmed for: 1.16.3, 1.16.4, 20w45a
@unknown, the ticket was already resolved as Won't Fix, thus we generally no longer add new affected versions to it.
@SPGoding I thought a mod mentioned in the Mojira subreddit this was internally marked for reassessment
I thought a mod mentioned in the Mojira subreddit this was internally marked for reassessment
internally being the key word here. This bug report remains resolved until Mojang decides to reopen it.

I've seen a lot of emails regarding new comments on this report.
You can discuss about this issue on the Mojira subreddit post: https://www.reddit.com/r/Mojira/comments/jhd3aa/why_was_mc26678_marked_as_wont_fix/
The thread linked above will soon be archived, as it will soon be 6 months old. I've created a proactive reddit thread so when the thread is inevitably archived, people can still comment their gripes without clogging up the comment section.

IT GOT FIXEDDDDDDDDDD
FINALLY! WOW! AMAZING!
Been watching this one for years. Thank you for finally fixing it. 🙂

only took 9 years

MC-265044 can probably be marked as related - both reports involve the camera not tilting under specific circumstances while they used to in previous versions.
Relates to MC-269238.

Also relates to MC-225335.