Index: includes/ajax.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/ajax.inc,v
retrieving revision 1.41
diff -u -p -r1.41 ajax.inc
--- includes/ajax.inc	23 Dec 2010 04:26:31 -0000	1.41
+++ includes/ajax.inc	25 Dec 2010 19:44:02 -0000
@@ -587,13 +587,24 @@ function ajax_pre_render_element($elemen
       case 'submit':
       case 'button':
       case 'image_button':
-        // Use the mousedown instead of the click event because form
-        // submission via pressing the enter key triggers a click event on
-        // submit inputs, inappropriately triggering AJAX behaviors.
+        // Pressing the ENTER key within a textfield triggers the click event of
+        // the form's first submit button. Triggering AJAX in this situation
+        // leads to problems, like breaking autocomplete textfields, so we bind
+        // to mousedown instead of click.
+        // @see http://drupal.org/node/216059
         $element['#ajax']['event'] = 'mousedown';
-        // Attach an additional event handler so that AJAX behaviors
-        // can be triggered still via keyboard input.
+        // Retain keyboard accessibility by setting 'keypress'. This causes
+        // ajax.js to trigger 'event' when SPACE or ENTER are pressed while the
+        // button has focus.
         $element['#ajax']['keypress'] = TRUE;
+        // Binding to mousedown rather than click means that it is possible to
+        // trigger a click by pressing the mouse, holding the mouse button down
+        // until the AJAX request is complete and the button is re-enabled, and
+        // then releasing the mouse button. Set 'prevent' so that ajax.js binds
+        // an additional handler to prevent such a click from triggering a
+        // non-AJAX form submission. This also prevents a textfield's ENTER
+        // press triggering this button's non-AJAX form submission behavior.
+        $element['#ajax']['prevent'] = 'click';
         break;
 
       case 'password':
Index: misc/ajax.js
===================================================================
RCS file: /cvs/drupal/drupal/misc/ajax.js,v
retrieving revision 1.35
diff -u -p -r1.35 ajax.js
--- misc/ajax.js	23 Dec 2010 04:26:31 -0000	1.35
+++ misc/ajax.js	25 Dec 2010 19:44:03 -0000
@@ -183,10 +183,17 @@ Drupal.ajax = function (base, element, e
   // can be triggered through keyboard input as well as e.g. a mousedown
   // action.
   if (element_settings.keypress) {
-    $(element_settings.element).keypress(function (event) {
+    $(ajax.element).keypress(function (event) {
       return ajax.keypressResponse(this, event);
     });
   }
+
+  // If necessary, prevent the browser default action of an additional event.
+  // For example, prevent the browser default action of a click, even if the
+  // AJAX behavior binds to mousedown.
+  if (element_settings.prevent) {
+    $(ajax.element).bind(element_settings.prevent, false);
+  }
 };
 
 /**