Nested animations interrupt ancestor animations

[tansongyang]

Describe the bug
When you initiate a nested animation, ancestor animations are interrupted and never resume. Not sure what the expected behavior is, but the current behavior seems like a bug.

To Reproduce
(Didn't see the instructions to clone a Twiddle until I started filling the issue. I had already made a GitHub repo.)

https://github.com/tansongyang/ember-animated-issue-repro

Steps to reproduce the behavior:

1. Clone repo and run ember s
2. Click on the "Render" button.
3. Before all child list items fade in, click "Begin Nested Animation" on any visible child.

Expected behavior
I expected that, once the child animation completed, the rest of the children would fade in. Currently, the page stops fading in the rest of the children - you'll only see the ones that were visible at the time you initiated the nested animation.

Screenshots

When I initiated the nested animation, only the first 8 items had rendered in. You can see that, after the nested animation finished (it's just a transition from "Begin Nested Animation" to "Animating..." - in retrospect that's a little confusing, because it looks like the nested animation isn't done, but it actually is), the rest of the items are stuck hidden.

Desktop (please complete the following information):

• OS: Windows
• Browser: Chrome
• Version: 81.0.4044.138

Additional context
The repro is a simplified version of a real bug we ran into in production.

[arthur5005]

I've experienced this myself a ton, all sorts of odd behaviour with nested animations, this included, and have avoided nesting animations all together.