The player model in the inventory is only updated once every tick, which makes the movement look choppy / laggy. The tick delta is not being passed to the entity render function. I have made a fix for this in the form of a fabric mod (relevant code here). For direct comparison, here is a video showing the inventory player model first with the mod enabled, then with it disabled.
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MC-178267 is an unrelated issue. The issue here is that the player model can't be updated any faster than 20 times a second.
This entire report is incorrect.
The model is rendered every frame. The fact that the delta is not passed is irrelevant, since the correct angle is calculated and passed in on every render. Your mod code in fact disables any interpolation from passing in the delta by setting the interpolation endpoints to the same values.
You can verify this by recording a high-FPS video of the game and looking at the individual frames. If what you were claiming was true, 6 frames in a row in any 120fps recording would be the same. This is not the case.
Maybe my explanation or attempt at a fix is not correct, but there is a very clear visual difference when the mod is enabled vs disabled, for example when looking at the leg animation when flying. (video)
Does MC-178267 describe your issue?