src/vs/workbench/contrib/preferences/browser/settingsTree.ts/getShowAddButtonList
function getShowAddButtonList(dataElement: SettingsTreeSettingElement, listDisplayValue: IListDataItem[]): boolean {
	if (dataElement.setting.enum && dataElement.setting.uniqueItems) {
		return dataElement.setting.enum.length - listDisplayValue.length > 0;
	} else {
		return true;
	}
}