fix: do not block navigation to current url (#19730)

Lets navigation cause server round-trip, when navigation is triggered by clicking a link.

Fixes #19635

flow-server/src/main/resources/com/vaadin/flow/server/frontend/Flow.tsx

@@ -170,7 +170,7 @@ function Flow() {
     });
     const {pathname, search, hash} = useLocation();
     const navigated = useRef<boolean>(false);
-
+    const fromAnchor = useRef<boolean>(false);
     const containerRef = useRef<RouterContainer | undefined>(undefined);
 
     const navigateEventHandler = useCallback((event: MouseEvent) => {
@@ -184,6 +184,9 @@ function Flow() {
         }
 
         navigated.current = false;
+        // When navigation is triggered by click on a link, fromAnchor is set to true
+        // in order to get a server round-trip even when navigating to the same URL again
+        fromAnchor.current = true;
         navigate(path);
     }, [navigate]);
 
@@ -233,10 +236,12 @@ function Flow() {
 
     useEffect(() => {
         if (blocker.state === 'blocked') {
-            if(navigated.current) {
+            // Do not skip server round-trip if navigation originates from a click on a link
+            if (navigated.current && !fromAnchor.current) {
                 blocker.proceed();
                 return;
             }
+            fromAnchor.current = false;
             const {pathname, search} = blocker.location;
             const routes = ((window as any)?.Vaadin?.routesConfig || []) as AgnosticRouteObject[];
             let matched = matchRoutes(Array.from(routes), window.location.pathname);
@@ -284,7 +289,7 @@ function Flow() {
     }, [blocker.state, blocker.location]);
 
     useEffect(() => {
-        if(navigated.current) {
+        if (navigated.current) {
             navigated.current = false;
             fireNavigated(pathname,search);
             return;

flow-tests/test-root-context/src/main/java/com/vaadin/flow/uitest/ui/NavigationEventsView.java

@@ -0,0 +1,69 @@
+/*
+ * Copyright 2000-2024 Vaadin Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+
+package com.vaadin.flow.uitest.ui;
+
+import com.vaadin.flow.component.html.Anchor;
+import com.vaadin.flow.component.html.Div;
+import com.vaadin.flow.dom.Element;
+import com.vaadin.flow.router.AfterNavigationEvent;
+import com.vaadin.flow.router.AfterNavigationObserver;
+import com.vaadin.flow.router.BeforeEnterEvent;
+import com.vaadin.flow.router.BeforeEnterObserver;
+import com.vaadin.flow.router.Route;
+import com.vaadin.flow.router.RouterLink;
+import com.vaadin.flow.uitest.servlet.ViewTestLayout;
+
+@Route(value = "com.vaadin.flow.uitest.ui.NavigationEventsView", layout = ViewTestLayout.class)
+public class NavigationEventsView extends Div
+        implements BeforeEnterObserver, AfterNavigationObserver {
+
+    public static final String BEFORE_ENTER = "BeforeEnterEvent";
+    public static final String AFTER_NAVIGATION = "AfterNavigationEvent";
+
+    private final Element messages = new Element("div");
+
+    public NavigationEventsView() {
+        messages.setAttribute("id", "messages");
+        getElement().appendChild(messages);
+
+        RouterLink routerLink = new RouterLink("RouterLink to self",
+                NavigationEventsView.class);
+        routerLink.setId("router-link");
+        Anchor anchor = new Anchor(
+                "/view/com.vaadin.flow.uitest.ui.NavigationEventsView",
+                "Anchor to self");
+        anchor.setId("anchor");
+
+        add(routerLink, anchor);
+    }
+
+    @Override
+    public void beforeEnter(BeforeEnterEvent event) {
+        addMessage(BEFORE_ENTER);
+    }
+
+    @Override
+    public void afterNavigation(AfterNavigationEvent afterNavigationEvent) {
+        addMessage(AFTER_NAVIGATION);
+    }
+
+    private void addMessage(String message) {
+        Element element = new Element("div");
+        element.setText(message);
+        messages.appendChild(element);
+    }
+}

flow-tests/test-root-context/src/test/java/com/vaadin/flow/uitest/ui/NavigationEventsIT.java

@@ -0,0 +1,65 @@
+/*
+ * Copyright 2000-2024 Vaadin Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package com.vaadin.flow.uitest.ui;
+
+import java.util.List;
+import java.util.stream.Collectors;
+
+import org.junit.Assert;
+import org.junit.Test;
+import org.openqa.selenium.By;
+import org.openqa.selenium.WebElement;
+
+import com.vaadin.flow.component.html.testbench.AnchorElement;
+import com.vaadin.flow.testutil.ChromeBrowserTest;
+
+public class NavigationEventsIT extends ChromeBrowserTest {
+
+    @Test
+    public void assertNavigationToSelfProducesNavigationEvents() {
+        open();
+
+        // Initially there should be one round of navigation events
+        assertMessages(2);
+
+        // RouterLink click should cause second set of events
+        $(AnchorElement.class).id("router-link").click();
+        assertMessages(4);
+
+        // Anchor click should cause third set of events
+        $(AnchorElement.class).id("anchor").click();
+        assertMessages(6);
+    }
+
+    private void assertMessages(int expectedSize) {
+        List<String> messages = getMessages();
+        Assert.assertEquals("Unexpected amount of navigation events",
+                expectedSize, messages.size());
+        Assert.assertEquals("Second to last event should be BeforeEnter",
+                NavigationEventsView.BEFORE_ENTER,
+                messages.get(expectedSize - 2));
+        Assert.assertEquals("Last event should be AfterNavigation",
+                NavigationEventsView.AFTER_NAVIGATION,
+                messages.get(expectedSize - 1));
+
+    }
+
+    private List<String> getMessages() {
+        return findElement(By.id("messages"))
+                .findElements(By.cssSelector("div")).stream()
+                .map(WebElement::getText).collect(Collectors.toList());
+    }
+}