Introduce moveBefore() state-preserving atomic move API

[domfarolino]

This PR introduces a new DOM API on the Node interface: moveBefore(). It mirrors insertBefore() in shape, but defers to a new DOM manipulation primitive that this PR adds in service of this new API: the "move" primitive. The move primitive contains some of the DOM tree bookkeeping steps from the remove primitive, as well as the insert primitive, and does three more interesting things:

1. Calls the moving steps hook with the moved node and the old parent (possibly null, just like the removing steps)
2. Queues a custom element callback reaction for the connectedMoveCallback()
3. Queues two back-to-back mutation record tasks: one for removal from the old parent; one for insertion into the new one

The power of the move primitive comes from the fact that the algorithm does not defer to the traditional insert and removal primitives, and therefore does not invoke the removing steps and insertion steps. This allows most state to be preserved by default (i.e., we don't tear down iframes, or close dialogs). Sometimes, the insertion/removing step overrides in other specifications have steps that do need to be performed during a move anyways. These specifications are expected to override the moving steps hook and perform the necessary work accordingly. See whatwg/html#10657 for HTML.

Remaining tasks (some will be PRs in other standards):

• Custom element integration
• Keep popovers open
• Don't call post-connection steps if state-preserving atomic move is in progress
• Don't call becomes connected / becomes browsing-context
• Only disconnect subframes on removal when state-preserving atomic move is not in progress
• Keep dialogs open: see removing steps
• img/source: this shouldn't count as a relevant mutation
• Preserve fullscreen
• Preserve focus
  • Need to resolve focusin event semantics
• ~[ ] Don't reset animations / transitions. See here
  • Maybe nothing needs to be done here. Given how element removals are handled, the spec does NOT require transitions to be removed from the UA's set of running transitions for moved nodes since they are never removed from the Document.~
• [ ] Preserve text-selection. See set the selection range. Edit: Nothing needs to be done here. Selection metadata (i.e., selectionStart and kin) is preserved by default in browsers, consistent with HTML (no action is taken on removal). The UI behavior of the selection not being highlighted is a side-effect of the element losing focus
• Selection API: don't reset the Document's selection
  • Updates to the selection range should happen according to how the DOM Standard primitives update ranges. The Selection API specification admits as much, by deferring to the insert and removal algorithms. Therefore, we should reference the move primitive from the Selection API specification, and ensure that the move primitive in this DOM Standard PR updates live ranges correctly.
  • selectionchange event: We've decided to allow selectionchange event to still fire, since it is queued in a task. No changes for this part are required.
• Pointer event state reset: see here
• Hide input/select picker: here
• Cancel pointer lock: here
• Containment: keep last remembered size(see here)

• At least two implementers are interested (and none opposed):
  • Chromium: https://issues.chromium.org/issues/40150299
  • WebKit: DOM State-preserving move (Node.prototype.moveBefore) WebKit/standards-positions#375
  • Gecko: DOM State-preserving move (Node.prototype.moveBefore) mozilla/standards-positions#1053
• Tests are written and can be reviewed and commented upon at:
  • https://github.com/web-platform-tests/wpt/tree/master/dom/nodes/moveBefore/tentative (need to mark as non-tentative)
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/40150299
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1923880
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=281223
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

---

Preview | Diff

[annevk]

I think the mutation record needs some more design work. I would expect it to capture the information of a remove and an insert at the same time. Perhaps it needs to be a new object, though we could further overload the existing MutationRecord as well I guess. At least I think you need:

• old target
• target
• moved node (I'm not sure you can ever move multiple at this point, but maybe we should allow for it in the mutation record design?)
• old previous sibling
• old next sibling
• previous sibling
• next sibling

Would be good to know what @smaug---- thinks and maybe @ajklein even wants to chime in.

[annevk]

I think it would be cleaner if we introduced "move" instead of overloading "insert", though I'm willing to be convinced. At least I always viewed this as introducing "move" as the third primitive following "insert" and "remove".

[domfarolino]

Yeah I think initially I saw it as a separate primitive too, but the more I looked at it, the more the difference between the two seemed really subtle. It mostly has to do with MutationObservers (and half of the relevant logic here is tucked away in the "remove" primitive) and not running the post-connection steps. So I feel like we'd end up with a near line-by-line copy of "insert", modulo one or two small differences. I'll take another look to see if my intuition is accurate, but I do kinda suspect this is where we'd end up.

[annevk]

It'd be good to have more details. Because you can't move a DocumentFragment so a vast number of checks don't apply. Adopt won't ever run.

There might be a number of range and shadow tree checks that end we end up duplicating, but perhaps that calls for abstracting those instead. At least to me a state flag seems rather unappealing.

[domfarolino]

It's possible there is enough from "insert" that we'd omit in "move", to make it creation worth it, sure.

At least to me a state flag seems rather unappealing.

With a separate "move" primitive and the decision to throw when we can't "move", we could get rid of the state variable that's currently in "insert" (i.e., <var>statePreservingAtomicMoveInProgress</var>). But the state bool on Document is what other specifications will refer to in their removal steps to react to a move appropriately, so I'm not sure we can get rid of that one.

[annevk]

Hmm, wouldn't we give other specifications "move steps"?

[domfarolino]

I thought about this a bit more. I think we're happy to create a new "move" primitive algorithm following the "insert" and "remove" algorithms. As you mentioned, it will be a simpler algorithm than "insert" since we'll avoid the DocumentFragment stuff — oh and also the post-connection stuff, among maybe more things.

There's still the discussion around introducing either (a) new "move" steps/hook that other specs provide to react to moves, vs (b) a Document-scoped boolean that the new "move" algorithm sets, for existing insertion/removal steps to use, to not reset certain state inside those steps.

Option (b) is the direction of the current PR, which we think makes sense after implementing this in Chromium. I think this is because so far we've found no behavior that we specifically want to do during a move that isn't done by insertion/removal — so any "move steps" we provided would just be empty. Does that make sense?

Now, I think the real question here is whether we want to:

1. Keep running the insertion/removal steps during a "move" operation, and just pepper if during atomic move is true, don't run some of these substeps! wherever state would ordinarily be reset
   • Cancel out the iframe removal steps
   • Cancel img relevant mutations, to avoid re-firing load events
2. Simply not run insertion/removal hooks at all during a move. This is only feasible if we determine that there are absolutely no steps in any spec's existing insertion/removal hooks that need to run during a move. We'll have to do an audit to determine this, which I'm starting now.

So the way I see it, those are probably our options, as opposed to creating new move steps which I think from our findings so far, would be unused. (We'll course-correct if this finding changes).

[smaug----]

• old target

• target

• moved node (I'm not sure you can ever move multiple at this point, but maybe we should allow for it in the mutation record design?)

Shouldn't the target node be all the time the same, it is just the siblings which change.
So we'd need only oldPreviousSibling and oldNextSibling. Oh, hmm, this isn't only about moving children but moving anything.

If this is really just remove and add back elsewhere, we could just reuse the existing childList MutationRecords, one for remove, one for adding node back, and possibly just add a flag to MutationRecord that it was about move.

(movedNodes is a bit confusing, since it seems to depend on the connectedness of the relevant nodes and it is apparently empty for the removal part. And it is unclear to me why we need the connectedness check. This is about basic DOM tree operations, and I'd assume those to work the same way whether or not the node is connected)

[annevk]

Creating two separate mutation records that a consumer would have to merge to (fully) understand it's a move seems suboptimal?

I agree that it should probably work for disconnected nodes as well, but I don't think we want to support a case where the shadow-including root changes.

[ajklein]

It's been a long time since I've thought about this stuff, but I'm inclined to agree with @smaug---- that creating a new type of MutationRecord feels unnecessary. Users of MutationObserver already have to do coalescing if they want to make sense of the stream of changes they observe. There are already other move-like operations, such as appending a child that's already somewhere else in the tree, which today generates two records.