The resizing icon allows you to change the dimensions
  of the window.

  Click on the icon, then, with the button held down,
  move the cursor outside the window. You will find an
  outline of the window follows the cursor, so that as
  you move the cursor, the outline demonstrates the new
  size and shape. When you release the button, the window
  will resize to the new shape.

  You can make the window smaller, by moving the cursor
  inside the current window, but you must first move the
  cursor outside to collect the outline. Then move it
  back inside the window border to shrink the window to
  your new requirements.