Just noticed this with the latest release, 15.1.4. I have a tabcontainer inside an updatepanel with updatemode = conditional. Some of the tabs trigger a modalpopupextender via code-behind also via child updatepanel. Initially, the modal popup and background display on top like they should, but after doing an update on the outer updatepanel, the modal popups and background css appear behind the tab container. z-index is -6000 for backgroundelement and -5999 for the modal popup when this happens. Used to work fine prior to release 15.1.4.
Comments: The issue does look like the one described in the [27101](https://ajaxcontroltoolkit.codeplex.com/workitem/27101) thread. It will be fixed in the next version. There is no solution for the current 15.1.4 version. Therefore, I recommend you downgrade AJAX Control Toolkit used in your project to the previous 15.1.3 version. This is the only way to overcome this issue until the next AJAX Control Tookit release. Would you please clarify if downgrading helps?
Comments: The issue does look like the one described in the [27101](https://ajaxcontroltoolkit.codeplex.com/workitem/27101) thread. It will be fixed in the next version. There is no solution for the current 15.1.4 version. Therefore, I recommend you downgrade AJAX Control Toolkit used in your project to the previous 15.1.3 version. This is the only way to overcome this issue until the next AJAX Control Tookit release. Would you please clarify if downgrading helps?