[MC-220635] Players can take fall damage with slow falling during lag spikes

The bug

Slow Falling normally negates fall damage, but if there is a lag spike timed around the point of reaching the ground, the player can still take fall damage. It seems to happen if the lag spike lasts long enough that the player falls 4+ blocks during the lag spike and reaches the ground as it ends.

I was able to consistently reproduce the bug by reloading while using a datapack that has a bunch of files and takes a couple seconds to load, but, importantly, nothing that affects Slow Falling.

How to reproduce

Give yourself Slow Falling
Type
```
/tp @s ~ ~10 ~
```
As you reach the ground, trigger a lag spike by typing
```
/reload
```
→ ❌ If timed correctly, you should take fall damage.

Attachments

2021-03-24 19-07-39.mp4

2021-03-24 19-08-30.mp4

Minecraft 21w11a - Singleplayer 2021-03-24 22-47-36.mp4

Comments 2

violine1101 2021-03-24T21:50:12Z

I'm unable to reproduce this.

[media]

I tried multiple times.

Aldinyx 2021-03-24T23:17:39Z

I've attached 2 videos showcasing the bug, one showing the bug from first person, and the other attempting to show that the Slow Falling effect is active the entire time (although the video lags a bit as its reloading which might look like a cut, but it's not).

I was having difficulty replicating it with just the vanilla datapack installed so I have another one with a bunch of files to create a larger lag spike but nothing that impacts the Slow Falling effect. The Slow Falling icon is present throughout both of the videos which should hopefully be sufficient enough to indicate this.