Thank you for the question. The Panel you are referring to is most likely the ToolWindow. If you place a ToolWIndow beside a DocumentWindow you can only make it floating again via the context menu that opens when you right click the tab. This is the default behavior in VS2005 and VS2008, however this was changed in VS2010, so we we are considering to change it as well
Please write again if you have other questions.
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