Fix BoundingBox.merge() to not modify receiver in place

Agent-Logs-Url: https://github.com/jMonkeyEngine/jmonkeyengine/sessions/e27f3192-530d-4525-aa2a-ad3e611ae4ed

Co-authored-by: riccardobl <4943530+riccardobl@users.noreply.github.com>

Author: Copilot

jme3-core/src/main/java/com/jme3/bounding/BoundingBox.java

@@ -404,18 +404,18 @@ public Plane.Side whichSide(Plane plane) {
     }
 
     /**
-     * <code>merge</code> combines this bounding box locally with a second
-     * bounding volume. The result contains both the original box and the second
-     * volume.
+     * <code>merge</code> combines this bounding box with a second bounding
+     * volume. The result contains both the original box and the second volume.
+     * The current instance is unaffected.
      *
      * @param volume the bounding volume to combine with this box (or null) (not
      * altered)
-     * @return this box (with its components modified) or null if the second
-     * volume is of some type other than AABB or Sphere
+     * @return a new BoundingBox, or null if the second volume is of some type
+     * other than AABB or Sphere
      */
     @Override
     public BoundingVolume merge(BoundingVolume volume) {
-        return mergeLocal(volume);
+        return clone(new BoundingBox()).mergeLocal(volume);
     }
 
     /**

jme3-core/src/test/java/com/jme3/bounding/TestBoundingBox.java

@@ -71,6 +71,34 @@ public void testEquals() {
         Assert.assertEquals(bb5, bb6); // because check planes are ignored
     }
 
+    /**
+     * Verify that merge() does not modify the original bounding box, and that
+     * the returned box contains both inputs. This was a bug: merge() used to
+     * call mergeLocal(), which modifies the receiver in place.
+     */
+    @Test
+    public void testMergeDoesNotModifyReceiver() {
+        BoundingBox bb1 = new BoundingBox(new Vector3f(0f, 0f, 0f), 1f, 1f, 1f);
+        BoundingBox bb2 = new BoundingBox(new Vector3f(4f, 0f, 0f), 1f, 1f, 1f);
+
+        // Record the original state of bb1 before merging.
+        BoundingBox bb1Before = (BoundingBox) bb1.clone();
+
+        BoundingVolume result = bb1.merge(bb2);
+
+        // bb1 must be unmodified.
+        Assert.assertEquals(bb1Before, bb1);
+
+        // The result must be a different object than bb1.
+        Assert.assertNotSame(bb1, result);
+
+        // The result must contain bb2's extent (i.e. the merged region).
+        Assert.assertTrue(result instanceof BoundingBox);
+        BoundingBox merged = (BoundingBox) result;
+        Assert.assertTrue(merged.contains(bb2.getCenter()));
+        Assert.assertTrue(merged.contains(bb1.getCenter()));
+    }
+
     /**
      * Verify that isSimilar() behaves as expected.
      */