fix: Fix JavaScriptBootstrapUI::getChildren

It should return all components added to the UI, like Dialog and Notification.
Related to #7799

Fixes #9069

(cherry picked from commit bae7bce)

flow-server/src/main/java/com/vaadin/flow/component/internal/JavaScriptBootstrapUI.java

@@ -73,12 +73,13 @@ public Stream<Component> getChildren() {
             return super.getChildren();
         }
 
-        // client-side routing,
-        // since virtual child is used, it is necessary to change the original
-        // UI element to the wrapperElement
+        // #9069 with client-side routing, since routing component is a virtual
+        // child, its children need to be included separately (there should only
+        // be one)
         Builder<Component> childComponents = Stream.builder();
         wrapperElement.getChildren().forEach(childElement -> ComponentUtil
                 .findComponents(childElement, childComponents::add));
+        super.getChildren().forEach(childComponents::add);
         return childComponents.build();
     }
 
@@ -221,7 +222,7 @@ void navigateToClient(String clientRoute) {
     private void acknowledgeClient() {
         serverConnected(false);
     }
-    
+
     private void cancelClient() {
         serverConnected(true);
     }

flow-server/src/test/java/com/vaadin/flow/component/internal/JavaScriptBootstrapUITest.java

@@ -1,24 +1,24 @@
 package com.vaadin.flow.component.internal;
 
-import static com.vaadin.flow.component.internal.JavaScriptBootstrapUI.CLIENT_NAVIGATE_TO;
-import static com.vaadin.flow.component.internal.JavaScriptBootstrapUI.SERVER_ROUTING;
-import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertFalse;
-import static org.junit.Assert.assertNull;
-import static org.junit.Assert.assertTrue;
-
 import java.util.Collections;
 import java.util.List;
 import java.util.Optional;
 import java.util.UUID;
 
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+import org.mockito.ArgumentCaptor;
+import org.mockito.Mockito;
+
 import com.vaadin.flow.component.Component;
 import com.vaadin.flow.component.HasComponents;
 import com.vaadin.flow.component.Tag;
 import com.vaadin.flow.component.UI;
 import com.vaadin.flow.component.page.AppShellConfigurator;
 import com.vaadin.flow.component.page.History;
 import com.vaadin.flow.component.page.Page;
+import com.vaadin.flow.dom.Element;
 import com.vaadin.flow.dom.impl.BasicElementStateProvider;
 import com.vaadin.flow.internal.CurrentInstance;
 import com.vaadin.flow.internal.StateNode;
@@ -31,15 +31,17 @@
 import com.vaadin.flow.router.Location;
 import com.vaadin.flow.router.PageTitle;
 import com.vaadin.flow.router.Route;
+import com.vaadin.flow.router.RouterLink;
 import com.vaadin.flow.server.InvalidRouteConfigurationException;
 import com.vaadin.flow.server.MockServletServiceSessionSetup;
 import com.vaadin.flow.server.VaadinRequest;
 
-import org.junit.Assert;
-import org.junit.Before;
-import org.junit.Test;
-import org.mockito.ArgumentCaptor;
-import org.mockito.Mockito;
+import static com.vaadin.flow.component.internal.JavaScriptBootstrapUI.CLIENT_NAVIGATE_TO;
+import static com.vaadin.flow.component.internal.JavaScriptBootstrapUI.SERVER_ROUTING;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertTrue;
 
 public class JavaScriptBootstrapUITest  {
 
@@ -204,6 +206,101 @@ public void getChildren_should_notReturnAnEmptyList() {
         assertEquals(1, ui.getChildren().count());
     }
 
+    @Test
+    public void addRemoveComponent_clientSideRouting_addsToBody() {
+        final Element uiElement = ui.getElement();
+
+        ui.connectClient("foo", "bar", "/clean", "");
+        // router outlet is a virtual child that is not reflected on element level
+        assertEquals(1, ui.getChildren().count());
+        assertEquals(0, uiElement.getChildCount());
+        assertEquals(0, ui.getElement().getChildCount());
+
+        final RouterLink routerLink = new RouterLink();
+        ui.add(routerLink);
+
+        assertEquals(2, ui.getChildren().count());
+        assertEquals(1, ui.getElement().getChildCount());
+        assertEquals(1, uiElement.getChildCount());
+
+        ui.add(new RouterLink());
+
+        assertEquals(3, ui.getChildren().count());
+        assertEquals(2, ui.getElement().getChildCount());
+        assertEquals(2, uiElement.getChildCount());
+
+        ui.remove(routerLink);
+
+        assertEquals(2, ui.getChildren().count());
+        assertEquals(1, ui.getElement().getChildCount());
+        assertEquals(1, uiElement.getChildCount());
+    }
+
+    @Test
+    public void addRemoveComponent_serverSideRouting_addsDirectlyToUI() {
+        Mockito.when(mocks.getSession().getAttribute(SERVER_ROUTING))
+                .thenReturn(Boolean.TRUE);
+
+        assertEquals(0, ui.getElement().getChildCount());
+        assertEquals(0, ui.getChildren().count());
+
+        // use any server side route
+        ui.navigate("product");
+
+        assertEquals(1, ui.getElement().getChildCount());
+        assertEquals(1, ui.getChildren().count());
+
+        final RouterLink routerLink = new RouterLink();
+        ui.add(routerLink);
+        assertEquals(2, ui.getElement().getChildCount());
+        assertEquals(2, ui.getChildren().count());
+
+        ui.remove(routerLink);
+
+        assertEquals(1, ui.getElement().getChildCount());
+        assertEquals(1, ui.getChildren().count());
+    }
+
+    @Test
+    public void addComponent_clientSideRouterAndNavigation_componentsRemain() {
+        final Element uiElement = ui.getElement();
+        // trigger route via client
+        ui.connectClient("foo", "bar", "/clean", "");
+        final RouterLink routerLink = new RouterLink();
+        ui.add(routerLink);
+
+        assertEquals(2, ui.getChildren().count());
+        assertEquals(1, ui.getElement().getChildCount());
+        assertEquals(1, uiElement.getChildCount());
+
+        ui.navigate("product");
+
+        assertEquals(2, ui.getChildren().count());
+        assertEquals(1, ui.getElement().getChildCount());
+        assertEquals(1, uiElement.getChildCount());
+    }
+
+    @Test
+    public void addComponent_serverSideRouterAndNavigation_componentsRemain() {
+        Mockito.when(mocks.getSession().getAttribute(SERVER_ROUTING))
+                .thenReturn(Boolean.TRUE);
+        final Element uiElement = ui.getElement();
+
+        ui.navigate("clean");
+        final RouterLink routerLink = new RouterLink();
+        ui.add(routerLink);
+
+        assertEquals(2, ui.getChildren().count());
+        assertEquals(2, ui.getElement().getChildCount());
+        assertEquals(2, uiElement.getChildCount());
+
+        ui.navigate("product");
+
+        assertEquals(2, ui.getChildren().count());
+        assertEquals(2, ui.getElement().getChildCount());
+        assertEquals(2, uiElement.getChildCount());
+    }
+
     @Test
     public void should_prevent_navigation_on_dirty() {
         ui.connectClient("foo", "bar", "/dirty", "");
@@ -452,7 +549,7 @@ public void should_not_share_dynamic_app_title_for_different_UIs(){
         String dynamicTitle = UUID.randomUUID().toString();
         ui.connectClient("foo", "bar", "clean", dynamicTitle);
         assertEquals(dynamicTitle, ui.getInternals().getTitle());
-        
+
         String anotherDynamicTitle = UUID.randomUUID().toString();
         JavaScriptBootstrapUI anotherUI = new JavaScriptBootstrapUI();
         anotherUI.getInternals().setSession(mocks.getSession());