disable-drupal-overlay.user.js

// ==UserScript==
// @name        Disable overlay
// @namespace   http://angrydan.example.com
// @description Disable the Drupal overlay
// @include     *
// @version     1
// @grant       none
// ==/UserScript==

// Drupal.ajaxError was introduced in D7. D6 won't have it.
if (typeof Drupal !== "undefined" && typeof Drupal.ajaxError !== 'undefined') {

		if (Drupal.behaviors.overlayParent) {
			delete Drupal.behaviors.overlayParent.attach;
		}

		// Following an AJAX call this function will be back, and Drupal will execute it.
		// So we do this now, to ensure we will be jumping the queue in future.
		Drupal.behaviors.preventOverlay = {
			attach: function(){
				if (Drupal.behaviors.overlayParent) {
					delete Drupal.behaviors.overlayParent.attach;
				}
			}
		}

}