mutation-observer-moved-subtree.window.js

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This WPT test may be referenced by the following Test IDs:
- /dom/nodes/moveBefore/mutation-observer-moved-subtree.window.html - WPT Dashboard Interop Dashboard

// META: title=moveBefore() and MutationObserver: a moved subtree remains observed until notification

"use strict";

promise_test(async t => {

  const root = document.createElement("div");

  const oldParent = root.appendChild(document.createElement("div"));

  const newParent = root.appendChild(document.createElement("div"));

  const target = oldParent.appendChild(document.createElement("div"));

  const observer = new MutationObserver(() => {});

  observer.observe(oldParent, { childList: true, subtree: true });

  newParent.moveBefore(target, null);

  const before = target.appendChild(document.createElement("span"));

  const records = observer.takeRecords();

  assert_equals(records.length, 2, "number of records");

  assert_equals(records[0].target, oldParent, "removal record target");

  assert_array_equals([...records[0].removedNodes], [target], "removal record removedNodes");

  assert_equals(records[1].target, target, "second record target");

  assert_array_equals([...records[1].addedNodes], [before], "second record addedNodes");

  // Mutation observers are notified in a microtask, so they have been notified once a new task runs.

  await new Promise(resolve => t.step_timeout(resolve, 0));

  target.append(document.createElement("span"));

  assert_array_equals(observer.takeRecords(), [], "records after mutation observers are notified");

}, "A subtree moved out of an observed subtree by moveBefore() is observed until mutation observers are notified");