Warn if using React.unmountComponentAtNode on a different React instance's tree.

[ventuno]

As #3819 has been closed, I tried to provide a fix for #3787.

1. If the node is a valid DOM node (e.g.: is of type Node.ELEMENT_NODE, Node.DOCUMENT_NODE or Node.DOCUMENT_FRAGMENT_NODE, see [1]);
2. and if the node has been rendered by React (e.g.: has the data-reactid or data-reactroot attribute, see Warn when using React.unmountComponentAtNode on a different React instance's tree #3787 (comment));
3. and if ReactDOMComponentTree.getInstanceFromNode returns null;
4. then, this element has been rendered by a different React instance and we show a warning.

[1] https://developer.mozilla.org/en-US/docs/Web/API/Node/nodeType

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[aweary]

@ventuno could you add tests for this as well to verify it works? Thanks!

[syranide]

Am I misunderstanding this or is the warning really confusing? It's not about unmounting something that "was rendered by another React instance", but trying to unmount something that isn't a React root (in this case, a React sub-tree). Right?

[aweary]

It's not about unmounting something that "was rendered by another React instance"

@syranide looking at #3787 that does seem to be the goal. From @spicyj in the google groups thread:

Due to our approach of storing internal state in React internals so that it's not accessible outside of React, we'll probably never support your use case of using React.unmountComponentAtNode with a different copy of React than the one that rendered it initially. We can add a warning for this though – opened #3787 to track.

[ventuno]

Indeed, the goal of this PR is to show a warning whenever a react node is being unmounted by a different React instance than the one that has originally rendered it.
I agree it's not a very common scenario, but it probably still makes sense to warn the user if such scenario is detectable as it generated confusion in the past (e.g.: the google groups thread mentioned by @aweary in #7456 (comment)).

[sophiebits]

Can we call this isValidContainer?

[sophiebits]

Thanks for sending this in! Looks good except a couple nits inline. Mind updating?

[sophiebits]

Thanks!