1 files changed, 45 insertions, 0 deletions
diff --git a/testing/web-platform/tests/html/webappapis/microtask-queuing/queue-microtask.window.js b/testing/web-platform/tests/html/webappapis/microtask-queuing/queue-microtask.window.js
new file mode 100644
index 0000000000..78cdcfc5d9
--- /dev/null
+++ b/testing/web-platform/tests/html/webappapis/microtask-queuing/queue-microtask.window.js
@@ -0,0 +1,45 @@
+"use strict";
+
+// This does not work as you expect because mutation observer compound microtasks are confusing.
+// Basically you can only use it once per test.
+function queueMicrotaskViaMO(cb) {
+  const observer = new MutationObserver(cb);
+  const node = document.createTextNode("");
+  observer.observe(node, { characterData: true });
+  node.data = "foo";
+}
+
+// Need to use promise_test to get sequential ordering; otherwise the global mutation observer
+// compound microtask business screws us over.
+
+promise_test(() => {
+  return new Promise(resolve => {
+    const happenings = [];
+
+    queueMicrotaskViaMO(() => happenings.push("x"));
+    queueMicrotask(() => happenings.push("a"));
+
+    queueMicrotask(() => {
+      assert_array_equals(happenings, ["x", "a"]);
+      resolve();
+    });
+  });
+}, "It interleaves with MutationObservers as expected");
+
+promise_test(() => {
+  return new Promise(resolve => {
+    const happenings = [];
+
+    queueMicrotask(() => happenings.push("a"));
+    Promise.reject().catch(() => happenings.push("x"));
+    queueMicrotaskViaMO(() => happenings.push(1));
+    Promise.resolve().then(() => happenings.push("y"));
+    queueMicrotask(() => happenings.push("b"));
+    queueMicrotask(() => happenings.push("c"));
+
+    queueMicrotask(() => {
+      assert_array_equals(happenings, ["a", "x", 1, "y", "b", "c"]);
+      resolve();
+    });
+  });
+}, "It interleaves with MutationObservers and promises together as expected");