To deactivate all tabs I have to set the ActiveTabIndex parameter to -1.

This was working up to version 4.6.0. After that version I start getting an error:

Unhandled exception rendering component: Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')
System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')

Here is an example with 4.6.0 where it is working:

https://blazorrepl.telerik.com/QSPuvKvI06m6AhF929

Here is an example with the current version:

https://blazorrepl.telerik.com/mIvOlqlI062NR9Vi59