code: @Override
      public void windowLostFocus(WindowEvent e) {
          // This section is part of the bug fix for blank popup windows in linux.
          if (!enableHideWhenFocusIsLost) {
              e.getWindow().requestFocus();
              return;
          }
          // This fixes a linux-specific behavior where the focus can be "lost" by clicking a child
          // component (inside the same panel!).
          if (InternalUtilities.isMouseWithinComponent(displayWindow)) {
              return;
          }
          hide();
      }
docs: windowLostFocus, Part of WindowFocusListener. Whenever the popup window loses focus, it will
  be hidden.