[Fabric LA] Fix maybeDropAncestors condition.

[bartlomiejbloniarz]

Summary

In the maybeDropAncestors function we can remove the view if it has no remaining animating views. Let's say we have nested exiting animations:

flowchart TD
    A((A: EXITING))
    B((B: EXITING))
    C((C: WAITING))
    D((D: WAITING))
    E((E: WAITING))

    A --> B
    A --> C
    A --> D
    A --> E

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In the current implementation in this case if the animation in B ended before A, we would visit A in maybeDropAncestors and decided to remove A, even though it still has some waiting children. Then A would be added to the view recycling pool while still having children. This would cause us to see some zombie views when the view is reused.

I changed the maybeDropAncestors condition to check the size of the children list. I also removed node->animatedChildren as I think it is no longer necessary.

Fixes #6644

Test plan

Chceck [LA] View recycling example, if there are no zombie views in the WheelPicker component.

[piaskowyk]

Smart 👍
Just to be sure if I understand correctly - We can simply rely on checking the count of children since they are detached before the parent, correct?

[bartlomiejbloniarz]

@piaskowyk We maintain the children count by observing mutations. We can remove the node only if there are no children left, as that means that all animations in the subtree have finished. The previous version was using the animatedChildren set, which actually didn't take all children into account, leading to us sometimes removing a node before its children.