fix(transition): transition-group should only listen for first-level children's end events

[verstoff]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

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• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
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• New/updated tests are included

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Other information:

If in your list you have transitions on the nested elements, they will trigger the transitionend events that will cause finishing of transition-group's transition.

[posva]

Could you provide a minimal repro of the bug please?

[verstoff]

@posva
Sure: codesandbox repro

p.s.
Of course, in our app code we can subscribe on child's events and use stopPropagation as a workaround. But I think, <transition-group> should only affect direct descendants
Or at least this case can be documented somewhere

[posva]

Thanks, now it is much clearer! This seems reasonable to me but could break some people work, so it would be interesting if there is any reasonable usage of nested elements transitions to trigger that transition-group. This is because, as you said, it's very easy to workaround the problem by adding @transitionend.stop to the nested element that also has a transition