Fix mounted event after initial hydration (#3480)

* trigger queued mounted events immediately after hydrating

* add a playwright test to prevent regressions

packages/web/src/lib.rs

@@ -125,6 +125,12 @@ pub async fn run(mut virtual_dom: VirtualDom, web_config: Config) -> ! {
 
             let rx = websys_dom.rehydrate(&virtual_dom).unwrap();
             hydration_receiver = Some(rx);
+
+            #[cfg(feature = "mounted")]
+            {
+                // Flush any mounted events that were queued up while hydrating
+                websys_dom.flush_queued_mounted_events();
+            }
         }
         #[cfg(not(feature = "hydrate"))]
         {