Thank you for writing. I am glad to hear that you managed to find the answer to your question.
At the time we introduced RadDock for WinForms only Visual Studio 2008 had its official version. While implementing RadDock we were following the behavior of this particular VS version and this is why you are not able to dock a ToolWindow (by a drag and drop operation) after it was previously placed in the documents area.
Not that Visual Studio 2010 is a fact, we can reconsider the current RadDock behavior. You can find a PITS item for this task here
I hope this helps.
All the best,
the Telerik team
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