docs: def paintEvent(self, event):
          """ Fills the panel background. """
          super(EncodingPanel, self).paintEvent(event)
          if self.isVisible():
              # fill background
              painter = QtGui.QPainter(self)
              self._background_brush = QtGui.QBrush(self._color)
              painter.fillRect(event.rect(), self._background_brush)