I used the code you provided to create a simple project and test the matter. I reproduced the problem and in my opinion it is due to the fact that the modal popup element is rendered inside the dock (speaking generally inside its parent). Since you want to achieve a modal behavior so that when the popup appears everything else be disabled, it is best practice for the popup element to be rendered directly as a child of the body element. When rendered inside a dock it needs to be rendered inside the element with the greatest stacking context
and your scenario does not fulfill this criteria (you can notice that if you have three docks and you place the modal popup extender in the second one, the first and the second docks get disabled, but third one does not). Therefore, we suggest that in this concrete scenario you try using radconfirm
. I tested your code with radconfirm and it works correctly.
the Telerik team