showPanel = (on, elem) => {
      $id('marker_panel').style.display = (on) ? 'block' : 'none'
      if (on && elem) {
        mtypes.forEach((pos) => {
          const marker = getLinked(elem, 'marker-' + pos)
          if (marker?.attributes?.se_type) {
            $id(`${pos}_marker_list_opts`).setAttribute('value', marker.attributes.se_type.value)
          } else {
            $id(`${pos}_marker_list_opts`).setAttribute('value', 'nomarker')
          }
        })
      }
    }
Toggles context tool panel off/on.
@param {boolean} on
@returns {void}