function openContextMenu(e, items) {
  e.preventDefault();
  e.stopPropagation();
  closeContextMenu();

  var overlay = document.createElement('div');
  overlay.className = 'cm-overlay';
  overlay.id = '_ctxMenuOverlay';
  overlay.onclick = function() { closeContextMenu(); };
  overlay.addEventListener('contextmenu', function(ev) {
    ev.preventDefault();
    ev.stopPropagation();
    var cx = ev.clientX, cy = ev.clientY;
    closeContextMenu();
    var el = document.elementFromPoint(cx, cy);
    if (el) el.dispatchEvent(new MouseEvent('contextmenu', {bubbles:true, cancelable:true, clientX:cx, clientY:cy, button:2}));
  });

  var menu = document.createElement('div');
  menu.className = 'cm-menu';
  menu.id = '_ctxMenu';
  menu.style.left = e.clientX + 'px';
  menu.style.top = e.clientY + 'px';
  menu.oncontextmenu = function(ev) { ev.preventDefault(); };

  function addBtn(label, cls, handler) {
    var btn = document.createElement('button');
    btn.textContent = label;
    if (cls) btn.className = cls;
    btn.onclick = function() { closeContextMenu(); handler(); };
    menu.appendChild(btn);
  }

  for (var i = 0; i < items.length; i++) {
    addBtn(items[i].label, items[i].cls || '', items[i].handler);
  }

  document.body.appendChild(overlay);
  document.body.appendChild(menu);
}

function closeContextMenu() {
  var o = document.getElementById('_ctxMenuOverlay');
  var m = document.getElementById('_ctxMenu');
  if (o) o.remove();
  if (m) m.remove();
}