Bug 1950487 [wpt PR 50945] - DOM: Add slotchange test for moveBefore(), a=testonly

Automatic update from web-platform-tests
DOM: Add slotchange test for moveBefore()

This CL adds a test to ensure that the 'slotchange' event is fired when
slots themselves are moved in and out of a custom element, and their
assigned nodes change.

This addresses
whatwg/dom#1307 (comment).

R=nrosenthal

Bug: 40150299
Change-Id: I93ee04294e5ab3e6d9f75c48705cdc77ce0a1df3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6298941
Commit-Queue: Dominic Farolino <domchromium.org>
Reviewed-by: Noam Rosenthal <nrosenthalchromium.org>
Cr-Commit-Position: refs/heads/main{#1424746}

--

wpt-commits: cd2cfbab2d2157b1c409be1631ff7e42c3ae7f6f
wpt-pr: 50945

UltraBlame original commit: 4a6a1072a3e50a2172f5d712305e8a77f170e452

Author: marco-c

testing/web-platform/tests/dom/nodes/moveBefore/tentative/slotchange-events.html

@@ -103,4 +103,37 @@
     await slotChangePromise;
   }
 }, "Moving a slottable into and out out of a custom element fires 'slotchange' event");
+
+promise_test(async t => {
+  const customElement = document.body.appendChild(document.createElement('custom-element'));
+  const slot = customElement.shadowRoot.children[0];
+  t.add_cleanup(() => customElement.remove());
+
+  const p = document.createElement('p');
+  p.slot = 'content';
+  p.textContent = 'The content';
+  customElement.appendChild(p);
+
+  // See the above tests that do the same thing, for implementations that do not fire `slotchange`
+  // at this phase.
+  await new Promise(resolve => t.step_timeout(() => resolve()));
+
+  assert_array_equals(slot.assignedNodes(), [p]);
+  document.body.moveBefore(slot, null);
+
+  await new Promise((resolve, reject) => {
+    slot.addEventListener('slotchange', e => resolve(), {once: true});
+    t.step_timeout(() => reject('Timeout; slotchange was not fired2'), 1500);
+  });
+
+  assert_array_equals(slot.assignedNodes(), []);
+  customElement.shadowRoot.moveBefore(slot, null);
+
+  await new Promise((resolve, reject) => {
+    slot.addEventListener('slotchange', e => resolve(), {once: true});
+    t.step_timeout(() => reject('Timeout; slotchange was not fired3'), 1500);
+  });
+
+  assert_array_equals(slot.assignedNodes(), [p]);
+}, "Moving a slot runs the assign slottables algorithm");
 </script>