Re: [whatwg/dom] Meta: a list of side effects that would be impacted by state-preserving move (Issue #1270)

> Mutation observer: potentially reflect that there was a move

It's already easy to fail behind MutationRecords order (added VS removed, which to check first? I check removed first and update the logic synchronously for libraries which goal is to reflect *connected* and *disconnected* behavior) so this would be awesome to tackle and I see only two ways to do so:

  * both removed and added nodes lists refer to the same node with an extra `moved` boolean value (safe as it's `undefined` and considered always `false` to date, if ever even checked)
  * movedNodes list enters the game with optionally a `from` and `to` references for the nodes

To be honest, this logic could be easily polyfilled too by checking the `parentNode` and `isConnected` properties of any removed node but if handled natively people would be surely happier ... the *move* operation is key to avoid tons of clean-up and events dispatching for something that maybe needs again, once reconnected, to bootstrap tons of stuff so it's not only the browser insertions/removals steps we all win, if less, it's a huge ecosystem of libraries that do a lot of work on disconnected and connected so I hope this MO related story will be as successful as the idea behind moving.

> Custom elements ... Perhaps add a movedCallback, that has a default to call disconnectedCallback and connectedCallback?

That would be the least surprise and fully backward compatible so I am overly :+1: with the movedCallback idea and its defaulting to current behavior: disconnected + connected

This helps Web Components libraries authors to migrate while browsers adoption increase so it feels like a no-brainer to me and I hope it will be welcomed by others too.

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