There is an old issue with hidden tabs, which is still exisitng in the new 15.1 release.
You will find the older issues by searching for "focus hidden".
The problem is limited to IE8 and doesn't occur on Firefox, IE10 or Chrome. I don't know if you still support IE8, but a fix will be easy if you do so.
AjaxControlToolkit\AjaxControlToolkit\Scripts\Tabs.js (line 695)
```
_setFocus: function(obj) {
$get("__tab_" + obj.get_element().id).focus();
},
```
A simple if-statement should fix the problem.
You will find the older issues by searching for "focus hidden".
The problem is limited to IE8 and doesn't occur on Firefox, IE10 or Chrome. I don't know if you still support IE8, but a fix will be easy if you do so.
AjaxControlToolkit\AjaxControlToolkit\Scripts\Tabs.js (line 695)
```
_setFocus: function(obj) {
$get("__tab_" + obj.get_element().id).focus();
},
```
A simple if-statement should fix the problem.