Let me get straight to your question. Basically we do not support such functionality and this sizing is deep involved with the design of the control. If you want to change it you will have to redesign the control. However, the common approach in such cases is to bind the ActualWidth
property of the control to the Width
property of the pop-up which shows the history list. Keep in mind that in your specific scenario this is not going to work because the control and the pop-up are in different visual trees and you can not control both of them.
Pavel R. Pavlov
the Telerik team