How to reproduce:
1. Install Telerik Controls (version: 2016.3.1024)
2. Launch Visual studio. Create Windows application project.
3. Drop RadPopupContainer from the toolbox on to the windows form. Save the form.
4. Mouse click on the control on design canvas to select RadScrollablePanelContainer panel.
5. Now click on smart tag of RadScrollablePanelContainer panel and select "Undock in parent container" option.
6. Now separate RadScrollablePanelContainer panel by moving it to some location on the form. Save the form.
7. Try to move RadPopupContainer. Observe that it does not allow to move.
8. Click on the smart tag of RadScrollablePanelContainer panel and select "Dock in parent" option. Observe that  the RadScrollablePanelContainer panel is not docked into the RadPopupContainer control.