DOM: reset target/relatedTarget

Helps with issue whatwg/dom#562 and PR whatwg/dom#585.

dom/events/relatedTargets.window.js

@@ -0,0 +1,50 @@
+const host = document.createElement("div"),
+      child = host.appendChild(document.createElement("p")),
+      shadow = host.attachShadow({ mode: "closed" }),
+      slot = shadow.appendChild(document.createElement("slot"));
+
+test(() => {
+  for (target of [shadow, slot]) {
+    for (relatedTarget of [new XMLHttpRequest(), self, host]) {
+      const event = new FocusEvent("demo", { relatedTarget: relatedTarget });
+      target.dispatchEvent(event);
+      assert_equals(event.target, null);
+      assert_equals(event.relatedTarget, null);
+    }
+  }
+}, "Reset if target pointed to a shadow tree");
+
+test(() => {
+  for (relatedTarget of [shadow, slot]) {
+    for (target of [new XMLHttpRequest(), self, host]) {
+      const event = new FocusEvent("demo", { relatedTarget: relatedTarget });
+      target.dispatchEvent(event);
+      assert_equals(event.target, null);
+      assert_equals(event.relatedTarget, null);
+    }
+  }
+}, "Reset if relatedTarget pointed to a shadow tree");
+
+async_test(t => {
+  const shadowChild = shadow.appendChild(document.createElement("div"));
+  shadowChild.addEventListener("demo", t.step_func(() => document.body.appendChild(shadowChild)));
+  const event = new FocusEvent("demo", { relatedTarget: new XMLHttpRequest() });
+  shadowChild.dispatchEvent(event);
+  assert_equals(shadowChild.parentNode, document.body);
+  assert_equals(event.target, null);
+  assert_equals(event.relatedTarget, null);
+  shadowChild.remove();
+  t.done();
+}, "Reset if target pointed to a shadow tree pre-dispatch");
+
+async_test(t => {
+  const shadowChild = shadow.appendChild(document.createElement("div"));
+  shadowChild.addEventListener("demo", t.step_func(() => document.body.appendChild(shadowChild)));
+  const event = new FocusEvent("demo", { relatedTarget: shadowChild });
+  document.body.dispatchEvent(event);
+  assert_equals(shadowChild.parentNode, document.body);
+  assert_equals(event.target, null);
+  assert_equals(event.relatedTarget, null);
+  shadowChild.remove();
+  t.done();
+}, "Reset if relatedTarget pointed to a shadow tree pre-dispatch");