title: border issue on frameless window with rounded edges (pyqt5)
body: <p>I made a frameless window and added a function to round the edges.
  However, when I add a border to the window trough a style sheet, the border doesn't show on the edges like shown <a href="https://prnt.sc/ou63i2" rel="nofollow noreferrer">here</a>.</p>
  
  <p>Minimal example:</p>
  
  <pre><code>class MainWindow(QtWidgets.QMainWindow):
      def __init__(self):
          super(MainWindow, self).__init__()
          self.initUI()
  
      def initUI(self):
          self.setWindowFlags(Qt.FramelessWindowHint)
          self.setFixedSize(300, 380)
          self.round_corners()
  
      def round_corners(self):
          radius = 9.0
          path = QtGui.QPainterPath()
          path.addRoundedRect(QtCore.QRectF(self.rect()), radius, radius)
          mask = QtGui.QRegion(path.toFillPolygon().toPolygon())
          self.setMask(mask)
  </code></pre>
  
  <p>Stylesheet: </p>
  
  <pre><code>QMainWindow
  {
      background: #002025;
      border: 1px solid #093038;
  }
  </code></pre>
  
  <p>I appreciate any ideas on how to fix that.</p>