Fix mounted event after initial hydration (#3480)

* trigger queued mounted events immediately after hydrating

* add a playwright test to prevent regressions

packages/playwright-tests/fullstack-mounted.spec.js

@@ -0,0 +1,10 @@
+// @ts-check
+const { test, expect } = require("@playwright/test");
+
+test("hydration", async ({ page }) => {
+  await page.goto("http://localhost:7777");
+
+  // Expect the page to contain the pending text.
+  const main = page.locator("#main");
+  await expect(main).toContainText("The mounted event was triggered.");
+});