src/java.desktop/macosx/classes/com/apple/laf/AquaComboBoxUI.java

*** 494,507 ****
      }
  
      // This is somewhat messy.  The difference here from BasicComboBoxUI.EnterAction is that
      // arrow up or down does not automatically select the
      @SuppressWarnings("serial") // anonymous class
!     private static final Action triggerSelectionAction = new AbstractAction() {
          public void actionPerformed(final ActionEvent e) {
              triggerSelectionEvent((JComboBox)e.getSource(), e);
          }
      };
  
      @SuppressWarnings("serial") // anonymous class
      private static final Action toggleSelectionAction = new AbstractAction() {
          public void actionPerformed(final ActionEvent e) {
--- 494,512 ----
      }
  
      // This is somewhat messy.  The difference here from BasicComboBoxUI.EnterAction is that
      // arrow up or down does not automatically select the
      @SuppressWarnings("serial") // anonymous class
!     private final Action triggerSelectionAction = new AbstractAction() {
          public void actionPerformed(final ActionEvent e) {
              triggerSelectionEvent((JComboBox)e.getSource(), e);
          }
+ 
+         @Override
+         public boolean isEnabled() {
+             return comboBox.isPopupVisible() && super.isEnabled();
+         }
      };
  
      @SuppressWarnings("serial") // anonymous class
      private static final Action toggleSelectionAction = new AbstractAction() {
          public void actionPerformed(final ActionEvent e) {