Thank you for writing.
Currently, RadControl for Winforms does not support this functionality and I was not able to implement it as a work around. The implementation of collapse / expand splitter buttons in our Demo application is custom one and is used only in the context of the Demo application. Feel free to use and adopt it in your application.
Nevertheless, the feature seems to be reasonable and I added it in our issue tracking system. We will consider implementing it in a future version if more people request the same functionality. Please follow this link
to add your vote and also subscribe to its status updates. We will do our best to implement this feature in one of the next releases.
Thank you for your time and cooperation. Your Telerik points have been updated.
the Telerik team