Hi Masayuki, can you please investigate if we still have something to do? Thank you.

Yeah, I checked that. Neither in m-c nor comm-central uses the legacy mutation events (except the debugger of the devtools). So, we can disable them now.

• https://searchfox.org/comm-central/search?q=DOMAttrModified&path=&case=false&regexp=false
• https://searchfox.org/comm-central/search?q=DOMCharacterDataModified&path=&case=false&regexp=false
• https://searchfox.org/comm-central/search?q=DOMNodeInserted&path=&case=false&regexp=false
• https://searchfox.org/comm-central/search?q=DOMNodeRemoved&path=&case=false&regexp=false
• https://searchfox.org/comm-central/search?q=DOMNodeInsertedIntoDocument&path=&case=false&regexp=false
• https://searchfox.org/comm-central/search?q=DOMNodeRemovedFromDocument&path=&case=false&regexp=false
• https://searchfox.org/comm-central/search?q=DOMSubtreeModified&path=&case=false&regexp=false

Oops, this is easier to check:
https://searchfox.org/comm-central/search?q=%28DOMAttrModified%7CDOMCharacterDataModified%7CDOMNodeInserted%7CDOMNodeRemoved%7CDOMNodeInsertedIntoDocument%7CDOMNodeRemovedFromDocument%7CDOMSubtreeModified%29&path=&case=false&regexp=true

Adding code to make EventDispatcher stop dispatching legacy mutation events causes performance regressions.
https://treeherder.mozilla.org/perfherder/comparesubtest?originalProject=try&newProject=try&newRevision=be77639da8710275f150c5b23a4658baa2bf0784&originalSignature=4586009&newSignature=4586009&framework=13&application=firefox&originalRevision=5d0f7fef10604ef28ec50abf311b24354152ce26&page=1&showOnlyImportant=1

Therefore, I think that what we can do here is that we can just make it check either a chrome document or a default handler is adding an event listener in EventListenerManager.

The legacy DOM mutation events shouldn't be used even in content and must not be
used in chrome.

This patch makes it crash if legacy mutation event listener is:

• added to a node in a chrome document
• added to a node in a native anonymous subtree
• added to the system event group
• implemented by C++

This causes some crash tests and chrome tests. Therefore, this patch modifies
them to use MutationObserver instead or removes them if it's impossible to
rewrite with MutationObserver.

Note that if I prevent to dispatch mutation events in some/all of the cases,
that causes performance regression in Speedometer3. Therefore, this patch
does not touch EventDispatcher.

https://hg.mozilla.org/mozilla-central/rev/992d6e7accc8