title: Password textfield gets cleared on toggling show/hide password button
body: <p>I have a UI that includes a password entry field and a show/hide button. A tester has pointed out the following inconsistent behaviour.</p>
  
  <p>If the password is hidden and half typed in (e.g. "abc") and the user hits the toggle button to show the password and continues typing then the new characters (e.g. "def") are added to the end of the initial entry (making "abcdef"). All well and good.</p>
  
  <p>However, if the password is shown and half typed in (e.g. "abc") and the user hits the toggle button to hide the password and continues typing then the new characters (e.g. "def") replace the initial entry (making "def"). So the show/hide toggle not only shows or hides the text but also changes the behaviour of the <code>UITextField</code> (append / clear and start over) when the next character is entered.</p>
  
  <p>Why is this happening?</p>