Ensure Fragment can merge attributes

Use a new map when merging as the original may be immutable.

Closes gh-34848

Author: rstoyanchev

spring-webflux/src/main/java/org/springframework/web/reactive/result/view/Fragment.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2024 the original author or authors.
+ * Copyright 2002-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -91,9 +91,7 @@ public void mergeAttributes(Model model) {
 		if (CollectionUtils.isEmpty(model.asMap())) {
 			return;
 		}
-		if (this.model == null) {
-			this.model = new LinkedHashMap<>();
-		}
+		this.model = new LinkedHashMap<>(this.model != null ? this.model : Collections.emptyMap());
 		model.asMap().forEach((key, value) -> this.model.putIfAbsent(key, value));
 	}
 

spring-webflux/src/test/java/org/springframework/web/reactive/result/view/FragmentTests.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2002-2025 the original author or authors.
+ *
+ * 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
+ *
+ *      https://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 org.springframework.web.reactive.result.view;
+
+import java.util.Map;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.ui.ConcurrentModel;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Unit tests for {@link Fragment}.
+ * @author Rossen Stoyanchev
+ */
+public class FragmentTests {
+
+	@Test
+	void mergeAttributes() {
+		Fragment fragment = Fragment.create("myView", Map.of("fruit", "apple"));
+		fragment.mergeAttributes(new ConcurrentModel("vegetable", "pepper"));
+
+		assertThat(fragment.model()).containsExactly(Map.entry("fruit", "apple"), Map.entry("vegetable", "pepper"));
+	}
+
+	@Test
+	void mergeAttributesCollision() {
+		Fragment fragment = Fragment.create("myView", Map.of("fruit", "apple"));
+		fragment.mergeAttributes(new ConcurrentModel("fruit", "orange"));
+
+		assertThat(fragment.model()).containsExactly(Map.entry("fruit", "apple"));
+	}
+
+}