Notes:
      1. This implementation includes two windows:
          a. An opener window (that is always open), and
          b. A popup window (that is opened from the opener window).
      2. To open the popup window, click on the opener window button
          that is entitled "About this JavaScript Program".
      3. To close the popup window (from the popup window itself)
          a. Click the mouse anywhere in the body of the popup window,
          b. Or click on the "Close" button.
      4. To close the popup window (from the opener window)
          click the mouse on the button "Close the About popup window".
      5. To change focus from the popup window to the opener window,
          click the mouse anywhere in the body of the opener window.
      6. To change focus from the opener window to the popup window,
          a. Click on the title bar of the popup window, or
          b. Click on the button entitled "About this JavaScript Program".
      7. When you close the opener window, the popup window (if it is
          currently open) is also automatically closed.