Thank you for your reply.
The observed behavior is the expected one. For comparison you can check that the Microsoft drop-down popup behaves in the same way. These popups have the top most style which makes them appear on top of all normal application windows. As a consequence they appear over the RadForm1. We will consider adding an option that will disable this behavior in the popup window.
As a work around you can close the RadRibbonBarGroup 's Popup manually when user click the item in the split button to open the RadForm1.For example:
private void radMenuItem1_Click(object sender, EventArgs e)
//open the form1
I hope this helps.
All the best,
the Telerik team