Qlik Sense extension: property panel dropdown with custom alternative

property_panel_dropdown_custom.js

width: {
  type: 'string',
  component: 'dropdown',
  label: 'Width',
  ref: 'width',
  options: [{value: '', label: 'Default'},
            {value: 'fill', label: 'Fill'},
            {value: 'custom',	label: 'Custom'}]
},
customwidth: {
  type: 'string',
  ref: 'customwidth',
  label: 'Custom width',
  expression: 'optional',
  show: function (data) {
    return data.width === 'custom';
  }
}