Spin off from bug 1617669

It seems that at the moment, nsMutationObserver set on a document will notify about nodes that are mutated in shadowDOM.

This happens because of https://searchfox.org/mozilla-central/rev/96f1457323cc598a36f5701f8e67aedaf97acfcf/dom/base/MutationObservers.cpp#84-88

It would be useful for use cases like Fluent (which requires callers to attach the root directly so shadowRoot will have its own MutationObserver) to be able to set an option in nsMutationObserver to make it not report such mutations.

Why not just checking SubtreeRoot(), or such in the mutation observer? seems it'd be pretty cheap.

DOMLocalization API allows you to connect any node as a root and then all its children should be observed (except of shadowDOM).

Maybe I could check SubtreeRoot and if it's shadowRoot then check if its one of the roots in DOMLocalization?

FWIW, I suggested use of SubtreeRoot in bug 1617669 ;)
https://phabricator.services.mozilla.com/D67111#inline-399583