Index: applications/editors/josm/plugins/reltoolbox/test/unit/relcontext/actions/ReconstructPolygonActionTest.java
===================================================================
--- applications/editors/josm/plugins/reltoolbox/test/unit/relcontext/actions/ReconstructPolygonActionTest.java	(revision 36136)
+++ applications/editors/josm/plugins/reltoolbox/test/unit/relcontext/actions/ReconstructPolygonActionTest.java	(revision 36136)
@@ -0,0 +1,143 @@
+// License: GPL. For details, see LICENSE file.
+package relcontext.actions;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.openstreetmap.josm.tools.I18n.tr;
+
+import java.awt.Component;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.Predicate;
+import java.util.stream.Stream;
+
+import javax.swing.JOptionPane;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.openstreetmap.josm.TestUtils;
+import org.openstreetmap.josm.actions.DeleteAction;
+import org.openstreetmap.josm.command.DeleteCommand;
+import org.openstreetmap.josm.data.osm.DataSet;
+import org.openstreetmap.josm.data.osm.DatasetConsistencyTest;
+import org.openstreetmap.josm.data.osm.OsmPrimitive;
+import org.openstreetmap.josm.data.osm.Relation;
+import org.openstreetmap.josm.data.osm.RelationMember;
+import org.openstreetmap.josm.data.osm.RelationToChildReference;
+import org.openstreetmap.josm.data.osm.Way;
+import org.openstreetmap.josm.gui.MainApplication;
+import org.openstreetmap.josm.gui.layer.OsmDataLayer;
+import org.openstreetmap.josm.gui.util.GuiHelper;
+import org.openstreetmap.josm.testutils.annotations.Main;
+import org.openstreetmap.josm.testutils.annotations.Projection;
+
+import junit.framework.AssertionFailedError;
+import mockit.Mock;
+import mockit.MockUp;
+import relcontext.ChosenRelation;
+
+/**
+ * Test class for {@link ReconstructPolygonAction}
+ */
+@Projection
+@Main
+class ReconstructPolygonActionTest {
+    private DataSet ds;
+    private ChosenRelation chosenRelation;
+    private Way way1;
+    private Way way2;
+    private Way way3;
+    private Relation relation;
+    private ReconstructPolygonAction action;
+
+    @BeforeEach
+    void setup() {
+        DeleteCommand.setDeletionCallback(new DeleteCommand.DeletionCallback() {
+            @Override
+            public boolean checkAndConfirmOutlyingDelete(Collection<? extends OsmPrimitive> primitives, Collection<? extends OsmPrimitive> ignore) {
+                return true;
+            }
+
+            @Override
+            public boolean confirmRelationDeletion(Collection<Relation> relations) {
+                return true;
+            }
+
+            @Override
+            public boolean confirmDeletionFromRelation(Collection<RelationToChildReference> references) {
+                return true;
+            }
+        });
+        ds = new DataSet();
+        MainApplication.getLayerManager().addLayer(new OsmDataLayer(ds, "ReconstructPolygonActionTest#testNonRegression23170", null));
+        chosenRelation = new ChosenRelation();
+        way1 = TestUtils.newWay("", TestUtils.newNode("name=1"), TestUtils.newNode("name=2"));
+        way2 = TestUtils.newWay("", way1.lastNode(), TestUtils.newNode("name=3"));
+        way3 = TestUtils.newWay("", way2.lastNode(), way1.firstNode());
+        relation = TestUtils.newRelation("type=multipolygon landuse=orchard",
+                new RelationMember("outer", way1),
+                new RelationMember("outer", way2),
+                new RelationMember("outer", way3));
+        ds.addPrimitiveRecursive(relation);
+        chosenRelation.set(relation);
+        action = new ReconstructPolygonAction(chosenRelation);
+    }
+
+    @AfterEach
+    void tearDown() {
+        DeleteCommand.setDeletionCallback(DeleteAction.defaultDeletionCallback);
+    }
+
+    /**
+     * Check that the reconstruct code works on a minimal level
+     */
+    @Test
+    void testPolygonReconstructSimple() {
+        assertDoesNotThrow(() -> action.actionPerformed(null));
+        assertTrue(relation.isDeleted());
+        assertEquals(2, Stream.of(way1, way2, way3).filter(Way::isDeleted).count());
+        final Way keptWay = Stream.of(way1, way2, way3).filter(w -> !w.isDeleted()).findFirst().orElseThrow(AssertionFailedError::new);
+        assertTrue(keptWay.isClosed());
+        assertEquals(4, keptWay.getNodesCount());
+        assertEquals(1, keptWay.getNodes().stream().distinct().filter(n -> "1".equals(n.get("name"))).count());
+        assertEquals(1, keptWay.getNodes().stream().distinct().filter(n -> "2".equals(n.get("name"))).count());
+        assertEquals(1, keptWay.getNodes().stream().distinct().filter(n -> "3".equals(n.get("name"))).count());
+        assertEmpty(DatasetConsistencyTest.runTests(ds));
+    }
+
+    /**
+     * Ensure that we bail if a way in the relation to be simplified will be deleted from another relation.
+     */
+    @Test
+    void testPolygonReconstructComplex() {
+        final AtomicReference<String> shownMessage = new AtomicReference<>();
+        new MockUp<JOptionPane>() {
+            @Mock
+            void showMessageDialog(Component parentComponent,
+                                   Object message, String title, int messageType) {
+                shownMessage.set((String) message);
+            }
+        };
+        final Relation otherRelation = TestUtils.newRelation("type=multipolygon landuse=retail",
+                new RelationMember("outer", way1),
+                new RelationMember("outer", way2),
+                new RelationMember("outer", way3),
+                new RelationMember("label", TestUtils.newNode("name=4")));
+        ds.addPrimitiveRecursive(otherRelation);
+        assertDoesNotThrow(() -> GuiHelper.runInEDTAndWait(() -> action.actionPerformed(null)));
+        assertEmpty(DatasetConsistencyTest.runTests(ds));
+        assertEquals(tr("Multipolygon must consist only of ways with one referring relation"), shownMessage.get());
+    }
+
+    /**
+     * Check that a string is empty
+     * @param string The string to check. Will be printed to log if not empty.
+     */
+    private static void assertEmpty(String string) {
+        assertTrue(string.isEmpty(), string);
+    }
+}