Index: trunk/test/unit/org/openstreetmap/josm/gui/widgets/HistoryComboBoxTest.java
===================================================================
--- trunk/test/unit/org/openstreetmap/josm/gui/widgets/HistoryComboBoxTest.java	(revision 18129)
+++ trunk/test/unit/org/openstreetmap/josm/gui/widgets/HistoryComboBoxTest.java	(revision 18131)
@@ -45,3 +45,25 @@
         assertDoesNotThrow(historyComboBox::addCurrentItemToHistory);
     }
+
+    /**
+     * Non-regression test for JOSM #21215: StackOverflowError
+     */
+    @Test
+    void testNonRegression21215() {
+        final HistoryComboBox historyComboBox = new HistoryComboBox();
+        // utils plugin2 added a listener that pretty much did this
+        historyComboBox.addItemListener(event -> historyComboBox.addCurrentItemToHistory());
+        final AutoCompletionItem testItem = new AutoCompletionItem("testNonRegression21215");
+        // Add the original item
+        historyComboBox.getEditor().setItem(testItem);
+        historyComboBox.addCurrentItemToHistory();
+
+        // add a new item
+        historyComboBox.getEditor().setItem(new AutoCompletionItem("testNonRegression21215_2"));
+        historyComboBox.addCurrentItemToHistory();
+
+        // Readd the first item
+        historyComboBox.getEditor().setItem(testItem);
+        assertDoesNotThrow(historyComboBox::addCurrentItemToHistory);
+    }
 }