This exact issue has been already discussed before with woritem [16321](https://ajaxcontroltoolkit.codeplex.com/workitem/16321). I was not able to find any other recent discussion about it.
I invite to read that report because symptoms are very well explained by the author.
I updated toolkit version from 30930 (Sep 2009) to 7.1213 (Dec 2013) on a project containing a complex implementation of TabContainer. Before the update I had no issue at all with that page and TabContainer in general, but as soon as I referenced new toolkit version I found out some strange behaviors.
I reduced it all in a very simple, standalone page, attached to this post.
Basically what happens is that when you set one or more tab to be invisible (no matter whether you do it by the attribute on tag declaration or in the code-behind), TabContainer starts to display mixed-content tabs after ActiveTabChanged event, when index requested is equals or greater than the one of the not visible.
Page attached shows it very clearly.
Note: none of the workarounds proposed in the old workitem is working in my environment.
Target framework 3.5, Toolkit version 7.1213, IIS 7.5, Windows 7 64 bit
Comments: I've run into the same issue. It's like the client-side code only knows about the visible tabs, but is using the server-side active index. Selecting the last tab (with one of the first tabs hidden) and performing a callback results in a javascript error: "Unable to get property '_set_active' of undefined or null reference" because it's using the server-side tab index (which accounts for ALL tabs, visible or not) to reference an element in an array that consists of only the tabs that are visible.
I invite to read that report because symptoms are very well explained by the author.
I updated toolkit version from 30930 (Sep 2009) to 7.1213 (Dec 2013) on a project containing a complex implementation of TabContainer. Before the update I had no issue at all with that page and TabContainer in general, but as soon as I referenced new toolkit version I found out some strange behaviors.
I reduced it all in a very simple, standalone page, attached to this post.
Basically what happens is that when you set one or more tab to be invisible (no matter whether you do it by the attribute on tag declaration or in the code-behind), TabContainer starts to display mixed-content tabs after ActiveTabChanged event, when index requested is equals or greater than the one of the not visible.
Page attached shows it very clearly.
Note: none of the workarounds proposed in the old workitem is working in my environment.
Target framework 3.5, Toolkit version 7.1213, IIS 7.5, Windows 7 64 bit
Comments: I've run into the same issue. It's like the client-side code only knows about the visible tabs, but is using the server-side active index. Selecting the last tab (with one of the first tabs hidden) and performing a callback results in a javascript error: "Unable to get property '_set_active' of undefined or null reference" because it's using the server-side tab index (which accounts for ALL tabs, visible or not) to reference an element in an array that consists of only the tabs that are visible.