function clickHandler(ev) {
  // only left clicks
  if (ev.button !== 0) return;

  var toggleEl = this;
  
  // exit if toggle button is disabled
  if (toggleEl.getAttribute('disabled') !== null) return;

  // prevent form submission
  ev.preventDefault();
  ev.stopPropagation();

  // toggle dropdown
  toggleDropdown(toggleEl);
}
Handle click events on dropdown toggle element.
@param {Event} ev - The DOM event