title: How can I make a transparent window stay on top?
body: <p>I'm trying to make a transparent window stays on top (z-order).</p>
  
  <p>I want to implement something like layer that indicates some parameters on the other program.</p>
  
  <p>So what I need is to make background transparent, not whole widget with QWidget::setWindowOpacity function.</p>
  
  <p>I found some documents about it</p>
  
  <ol>
  <li><p><a href="http://doc.qt.io/archives/qq/qq16-background.html" rel="nofollow noreferrer">Transparent Backgrounds in Qt 4.1</a></p></li>
  <li><p><a href="http://doc.qt.io/archives/4.6/qwidget.html#transparency-and-double-buffering" rel="nofollow noreferrer">QWidget Class Reference — Transparency and Double Buffering</a></p></li>
  </ol>
  
  <p><strong>The first one</strong> explains "New in Qt 4.1: The Backing Store" and the picture is exactly what I want.</p>
  
  <p><strong>The second one</strong> explains more details about it. (left picture)</p>
  
  <blockquote>
    <p>The left widget has no additional properties or widget attributes set. This default state suits most custom widgets using transparency, are irregularly-shaped, or do not paint over their entire area with an opaque brush.</p>
  </blockquote>
  
  <p>But I don't know how to do it. I'm using Qt 4.6 B.T.W.</p>