Index: src/org/openstreetmap/josm/gui/tagging/ac/AutoCompletingComboBox.java
===================================================================
--- src/org/openstreetmap/josm/gui/tagging/ac/AutoCompletingComboBox.java	(revision 14977)
+++ src/org/openstreetmap/josm/gui/tagging/ac/AutoCompletingComboBox.java	(working copy)
@@ -297,6 +297,7 @@
         for (String elem : elems) {
             model.addElement(new AutoCompletionItem(elem, AutoCompletionPriority.UNKNOWN));
         }
+        this.setSelectedItem(null);
         // disable autocomplete to prevent unnecessary actions in AutoCompletingComboBoxDocument#insertString
         autocompleteEnabled = false;
         this.getEditor().setItem(oldValue); // Do not use setSelectedItem(oldValue); (fix #8013)