code: def _get_margin_width(self, cli, margin):
          
          # Margin.get_width, needs to have a UIContent instance.
          def get_ui_content():
              return self._get_ui_content(cli, width=0, height=0)
  
          def get_width():
              return margin.get_width(cli, get_ui_content)
  
          key = (margin, cli.render_counter)
          return self._margin_width_cache.get(key, get_width)
docs: Return the width for this margin.
          (Calculate only once per render time.)