TabbedWindow with 2 tabs, each tab has Frame which almost fully covers the content.
Step 1) Drag drop a tab to the visible content. 2) Then bring it back to the first window with drag drop. Step 1 is now impossible - no window is created after drag out of tab strip and drop onto the content.
Possible workaround: Set AllowDrop to True to the control (in this case frame) in the content of the RadTabItems.