Fix onmounted event triggering many times (#3346)

* Fix event handler diffing

* Add headless tests for the onmounted event

* fix typo triggerd -> triggered

packages/playwright-tests/web.spec.js

@@ -107,3 +107,11 @@ test("prevent default", async ({ page }) => {
   // Check that the <a> element changed.
   await expect(a).toHaveText("Psych!");
 });
+
+test("onmounted", async ({ page }) => {
+  await page.goto("http://localhost:9999");
+
+  // Expect the onmounted event to be called exactly once.
+  const mountedDiv = page.locator("div.onmounted-div");
+  await expect(mountedDiv).toHaveText("onmounted was called 1 times");
+});