Index: wp-includes/js/quicktags.dev.js
===================================================================
--- wp-includes/js/quicktags.dev.js	(revision 21417)
+++ wp-includes/js/quicktags.dev.js	(working copy)
@@ -182,8 +182,12 @@
 		// listen for click events
 		onclick = function(e) {
 			e = e || window.event;
-			var target = e.target || e.srcElement, i;
+			var target = e.target || e.srcElement, visible = target.clientWidth || target.offsetWidth, i;
 
+			// don't call the callback on pressing the accesskey when the button is not visible
+			if ( !visible )
+				return;
+
 			// as long as it has the class ed_button, execute the callback
 			if ( / ed_button /.test(' ' + target.className + ' ') ) {
 				// we have to reassign canvas here