docs: def _draw_visible_area(self, painter):
          """
          Draw the visible area.
  
          This method does not take folded blocks into account.
  
          :type painter: QtGui.QPainter
          """
          if self.editor.visible_blocks:
              start = self.editor.visible_blocks[0][-1]
              end = self.editor.visible_blocks[-1][-1]
              rect = QtCore.QRect()
              rect.setX(0)
              rect.setY(start.blockNumber() * self.get_marker_height())
              rect.setWidth(self.sizeHint().width())
              rect.setBottom(end.blockNumber() * self.get_marker_height())
              if self.editor.background.lightness() < 128:
                  c = self.editor.background.darker(150)
              else:
                  c = self.editor.background.darker(110)
              c.setAlpha(128)
              painter.fillRect(rect, c)