Adopt node before removing child in replace

[annevk]

replace removes child (with suppressObservers set) before inserting node. Because insert now adopts node as one of its steps, when node is already in the tree its removal from the old location — and the mutation record that removal queues — happened after child had already been silently removed. That gave the record a stale previousSibling and made the two records appear non-atomic.

Adopt node up front, before child is removed, so node's removal is observed while child is still present. This restores the record ordering and previousSibling that predates #754, which had removed replace's explicit adoption step without accounting for this interaction. All engines kept the pre-#754 behavior, so this aligns the standard with reality.

The related replace all algorithm intentionally keeps adopting inside insert: its only caller that can receive an already-present node is replaceChildren(), which postdates #754 and folds such a node into its single record.

Fixes #814.

• At least two implementers are interested (and none opposed):
  • All engines already implement this behavior (it predates Run adopt as part of insert #754); this change aligns the standard with them.
• Tests are written and can be reviewed and commented upon at:
  • Existing coverage in dom/nodes/MutationObserver-childList.html (n52/n53) already asserts this behavior.
  • Add MutationObserver replaceChildren childList tests web-platform-tests/wpt#61155 adds related replaceChildren() coverage.
• Implementation bugs are filed:
  • N/A
• MDN issue is filed: N/A
• The top of this comment includes a clear commit message to use.

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

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[domfarolino]

Nice!