docs: def contextMenuEvent(self, event):
          """
          Add menu action:
          * 'Show line numbers'
          * 'Save to file'
          """
          menu = QtWidgets.QTextEdit.createStandardContextMenu(self)
  
          # create max.lines spin box:
          w = QtWidgets.QWidget()
          l = QtWidgets.QHBoxLayout()
          w.setLayout(l)
          e = QtWidgets.QSpinBox()
          e.setRange(1, 1e6)
          e.setValue(self.MAXLINES)
          e.valueChanged.connect(self.document().setMaximumBlockCount)
          l.addWidget(QtWidgets.QLabel('Max. lines'))
          l.addWidget(e)
  
          # add spinbox to menu:
          a = QtWidgets.QWidgetAction(self)
          a.setDefaultWidget(w)
          menu.addAction(a)
  
          menu.exec_(event.globalPos())