title: QPushButton setEnabled(false) does not gray out the button with lambda
body: <p>I discovered a very curious thing. 
  I have a big MainWindow class that does a lot of computations with a GUI. In order to keep the GUI responsive, I created a thread for a QTimer that updates the GUI every <em>n</em> milliseconds and another thread for the computations, and now all is very smooth and fast.</p>
  
  <p>A simple problem I noticed is that when calling this</p>
  
  <pre><code>connect (_worker, &amp;RRTWorker::finished, [=]()
  {
      ui-&gt;stopButton-&gt;setEnabled(false);
  });
  </code></pre>
  
  <p>, that is, when the <code>worker</code> emits the signal <code>finished</code> (when it has ended computations), both with <code>[=]</code> and <code>[&amp;]</code> the <code>stopButton</code> gets disabled <strong>BUT</strong> the label inside is still black (not with that greyish filter as normal disabled buttons).
  If you need some feedback, here is a picture:
  <a href="https://i.stack.imgur.com/DZx0w.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/DZx0w.png" alt="Bad disabled Stop button"></a></p>
  
  <p>As you can clearly see, the Start button gets grayed out completely, and that is done in the <code>on_startButtonclicked()</code> via the statement
  <code>ui-&gt;startButton-&gt;setEnabled(false)</code>.
  So, is this a bug, or am I doing something wrong?</p>