Closes #16997

Just an experiment...the comment was proposing overloading context but I think that would be bad for two reasons:

1. it's an overload which is never nice
2. You can technically do stuff that is not just setting context in this function (I don't know if you should but you can)

This could also have some interesting side effects like being able to register an effect that is in context of the component but outside of it...dunno maybe could be useful for testing.

Didn't add a test because we don't have test for mount...but...should we?

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