title: Qt transparent label doesn't always update its background
body: <p>I have 3 child widgets of the main widget which are aimed to play video files(their sizes and positions are the same, I mean they are overlaying each other and at a time one of them is shown and others are hidden)</p>
  
  <p>I also have a Qlabel with a transparent background that I created as a child of main widget as well. I raised this label in order to be shown over 3 video player widgets all the time. The code part of Qlabel is below:</p>
  
  <pre><code>labelCamName = new QLabel(this);
  labelCamName-&gt;resize(300,50);
  labelCamName-&gt;move(110,0);
  
  labelCamName-&gt;setStyleSheet("background-color: rgba(0,0,0,0%);color: rgba(150, 30, 30, 90%)");
  
  labelCamName-&gt;setText("blabla");
  labelCamName-&gt;raise();
  labelCamName-&gt;show();
  </code></pre>
  
  <p>It works fine when the first video player widget that the label has been overlayed update itself(playing a video) but when it comes to change the widget playing video and show another widget to be played(not playing yet) that it shows a stationary black frame, the background of the label shows the last frame of the first video player widget. It doesn't pretend like it is a label with transparent backgroud, as shown by screenshots below:</p>
  
  <p><img src="https://i.stack.imgur.com/O39GR.jpg" alt="first video Widget"></p>
  
  <p><img src="https://i.stack.imgur.com/W0YAV.jpg" alt="second video player widget"></p>
  
  <p>Any suggestion why Im facing this issue and how to make the label work always transparent? </p>
  
  <p>Thanks in advance. Very appreciated. </p>