Index: trunk/test/unit/org/openstreetmap/josm/tools/GeometryTest.java
===================================================================
--- trunk/test/unit/org/openstreetmap/josm/tools/GeometryTest.java	(revision 19445)
+++ trunk/test/unit/org/openstreetmap/josm/tools/GeometryTest.java	(revision 19446)
@@ -37,4 +37,5 @@
 import org.openstreetmap.josm.data.osm.RelationMember;
 import org.openstreetmap.josm.data.osm.Way;
+import org.openstreetmap.josm.data.osm.WaySegment;
 import org.openstreetmap.josm.data.osm.search.SearchCompiler;
 import org.openstreetmap.josm.data.projection.Projection;
@@ -636,3 +637,27 @@
         assertTrue(new EastNorth(0, 0).equalsEpsilon(centroid3, 1e-9));
     }
+
+    /**
+     * A non-regression test for an issue found in #24485
+     * Test of {@link Geometry#getClosestWaySegment} method when DataSet is read-only.
+     */
+    @Test
+    void testNonRegression24485() {
+        Node node1 = new Node(new LatLon(0, 0));
+        Node node2 = new Node(new LatLon(0, 1));
+        Node node3 = new Node(new LatLon(0.3, 0.5));
+        Node node4 = new Node(new LatLon(0.1, 0));
+        Way way1 = TestUtils.newWay("", node1, node2, node3, node4);
+
+        DataSet ds = new DataSet();
+        way1.getNodes().forEach(n -> ds.addPrimitive(n));
+        ds.addPrimitive(way1);
+        ds.lock();
+        WaySegment closestSegment = Geometry.getClosestWaySegment(way1, new Node(new LatLon(0, 0.5)));
+        ds.unlock();
+        Way closestSegmentWay = closestSegment.toWay();
+        assertTrue(closestSegmentWay.containsNode(node1));
+        assertTrue(closestSegmentWay.containsNode(node2));
+    }
+
 }