code: function tab( event ) {
  
    // Find all focusable elements in the dialog card
    var focusable   = dialogEl.querySelectorAll('input:not([type=hidden]), textarea, select, button'),
        last        = focusable[focusable.length - 1],
        focused     = document.activeElement
  
    // If focused on the last focusable element, tab to the first element.
    if ( focused == last ) {
      if ( event ){ event.preventDefault() }
      focusable[0].focus()
    }
  
    // Focus on the last element if the focused element is not a child of the dialog
    if ( !focused || !toolbox.childOf( focused, dialogEl ) ) {
      last.focus()
    }
  }
docs: In a modal window, `tab` must cycle between elements in the modal.