code: function _updateMask ( type, x, y, w, h ) {
    if ( !_dockMask ) {
      // add dock mask
      _dockMask = document.createElement('div');
      _dockMask.classList.add('dock-mask');
      _dockMask.oncontextmenu = function() { return false; };
    }
  
    if ( type === 'dock' ) {
      _dockMask.classList.remove('tab');
      _dockMask.classList.add('dock');
    } else if ( type === 'tab' ) {
      _dockMask.classList.remove('dock');
      _dockMask.classList.add('tab');
    }
  
    _dockMask.style.left = `${x}px`;
    _dockMask.style.top = `${y}px`;
    _dockMask.style.width = `${w}px`;
    _dockMask.style.height = `${h}px`;
  
    if ( !_dockMask.parentElement ) {
      document.body.appendChild(_dockMask);
    }
  }
docs: ========================== Internal ==========================