When the inventory is open while you are flying with an elytra, the player model in the inventory appear very weird and sometimes upside-down and/or outside of the boundaries. The player in the inventory also follows wherever the mouse curser is, and rotates when facing different ways in the world.
I also searched for this in the bug tracker, and I found someone else mentioning this back in 1.9. On there it said it was resolved, even though it isn't.
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Thank you for your report!
We're tracking this issue in MC-85582, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
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