fix: improve locationPrefix impl (#10719)

fixes #4730

flow-server/src/main/java/com/vaadin/flow/router/HighlightConditions.java

@@ -45,8 +45,9 @@ public static HighlightCondition<RouterLink> sameLocation() {
      * @return the highlight condition
      */
     public static HighlightCondition<RouterLink> locationPrefix() {
-        return (link, event) -> event.getLocation().getPath()
-                .startsWith(link.getHref());
+        return (link, event) -> link.getHref().isEmpty()
+                ? event.getLocation().getPath().isEmpty()
+                : event.getLocation().getPath().startsWith(link.getHref());
     }
 
     /**

flow-server/src/test/java/com/vaadin/flow/router/HighlightConditionsTest.java

@@ -0,0 +1,75 @@
+/*
+ * Copyright 2000-2021 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.router;
+
+import org.junit.Assert;
+import org.junit.Test;
+import org.mockito.Mockito;
+
+public class HighlightConditionsTest {
+
+    @Test
+    public void locationPrefix_defaultRoute_emptyLocationMatches() {
+        HighlightCondition<RouterLink> condition = HighlightConditions
+                .locationPrefix();
+        RouterLink link = Mockito.mock(RouterLink.class);
+        AfterNavigationEvent event = Mockito.mock(AfterNavigationEvent.class);
+
+        Mockito.when(link.getHref()).thenReturn("");
+        Location location = new Location("");
+        Mockito.when(event.getLocation()).thenReturn(location);
+        Assert.assertTrue(condition.shouldHighlight(link, event));
+    }
+
+    @Test
+    public void locationPrefix_defaultRoute_nonEmptyLocationDoesNotMatch() {
+        HighlightCondition<RouterLink> condition = HighlightConditions
+                .locationPrefix();
+        RouterLink link = Mockito.mock(RouterLink.class);
+        AfterNavigationEvent event = Mockito.mock(AfterNavigationEvent.class);
+
+        Mockito.when(link.getHref()).thenReturn("");
+        Location location = new Location("foo");
+        Mockito.when(event.getLocation()).thenReturn(location);
+        Assert.assertFalse(condition.shouldHighlight(link, event));
+    }
+
+    @Test
+    public void locationPrefix_notDefaultRoute_prefixMatches() {
+        HighlightCondition<RouterLink> condition = HighlightConditions
+                .locationPrefix();
+        RouterLink link = Mockito.mock(RouterLink.class);
+        AfterNavigationEvent event = Mockito.mock(AfterNavigationEvent.class);
+
+        Mockito.when(link.getHref()).thenReturn("foo");
+        Location location = new Location("foobar");
+        Mockito.when(event.getLocation()).thenReturn(location);
+        Assert.assertTrue(condition.shouldHighlight(link, event));
+    }
+
+    @Test
+    public void locationPrefix_notDefaultRoute_nonPrefixDoesNotMatch() {
+        HighlightCondition<RouterLink> condition = HighlightConditions
+                .locationPrefix();
+        RouterLink link = Mockito.mock(RouterLink.class);
+        AfterNavigationEvent event = Mockito.mock(AfterNavigationEvent.class);
+
+        Mockito.when(link.getHref()).thenReturn("foo");
+        Location location = new Location("bar");
+        Mockito.when(event.getLocation()).thenReturn(location);
+        Assert.assertFalse(condition.shouldHighlight(link, event));
+    }
+}