code: def _currentLineExtraSelections(self):
          
          if self._currentLineColor is None:
              return []
  
          def makeSelection(cursor):
              selection = QTextEdit.ExtraSelection()
              selection.format.setBackground(self._currentLineColor)
              selection.format.setProperty(QTextFormat.FullWidthSelection, True)
              cursor.clearSelection()
              selection.cursor = cursor
              return selection
  
          rectangularSelectionCursors = self._rectangularSelection.cursors()
          if rectangularSelectionCursors:
              return [makeSelection(cursor) \
                          for cursor in rectangularSelectionCursors]
          else:
              return [makeSelection(self.textCursor())]
docs: QTextEdit.ExtraSelection, which highlightes current line