[BUG] AnimatePresence & staggerChildren doesn't work when only some children are leaving and entering at the same time.

[agonsalves]

1. Describe the bug
When you have AnimatePresence wrapped around a motion.div that has the staggerChildren transition, the initial rendering of the entire block works. However, if you remove some of the children, they do not use their exit animation. Then, when you add them back, they all come back at once.

2. Provide a CodeSandbox reproduction of the bug
https://codesandbox.io/embed/wonderful-dubinsky-75s3n

3. Steps to reproduce
Just use the toggle in the code sandbox

4. Expected behavior
I expect the exit animation to happen when the items unmount. I also expect that when the items are mounted again, they are staggered.

[msheakoski]

This was also brought up in #294. Linking here because you have a good Code Sandbox example.

[mattgperry]

AnimatePresence isn't going to work in this instance because its root child never leaves. This isn't a bug but improper use of the API.

When the children enter again, the parent isn't animating, so it can't stagger its children. This is a trickier one to solve and I don't see a fix on the horizon that would work as expected in all situations, I would recommend using a custom delay strategy here.