title: Centralize a Frame in Another Frame in QT Designer
body: <p>In the first picture I designed a prototype. The <code>MainWindow</code> includes widget and three frames. The center frame has another frame, and when I run in the application in full screen there is a size problem in the center frame, as you can see in the second picture. I want to center the frame in its frame. How can I do that?</p>
  
  <p><img src="https://i.stack.imgur.com/nqEXc.png" alt="first picture">
  <img src="https://i.stack.imgur.com/cUG5u.png" alt="second picture"></p>