Do you have IFRAMEs in your splitter (or you are displaying external pages there)? If this is so, this is the reason for the behavior that you experience. When RadWindow is being moved or resized we hide all IFRAMEs on the page to prevent the cursor from going over one of them. If this happens, the IFRAME element consumes the mouse events and if this happens RadWindow will not "know" when the moving / resizing is finished.
If this is not your case, please open a support ticket and send me a small sample project where I can observe the problem. Just make sure that the project can be run locally and attach it to the support thread - I will check it right away.
the Telerik team