Divide the focus edit widget at the cursor location.

def split_focus(self):
        """Divide the focus edit widget at the cursor location."""

        focus = self.lines[self.focus]
        pos = focus.edit_pos
        edit = urwid.Edit("", focus.edit_text[pos:], allow_tab=True)
        edit.original_text = ""
        focus.set_edit_text(focus.edit_text[:pos])
        edit.set_edit_pos(0)
        self.lines.insert(self.focus + 1, edit)