PROBLEM The height of the DockableObject increases when collapsing and expanding it. DESCRIPTION This problem occurs when particular DOCTYPEs are used. The <!DOCTYPE> declaration is the very first thing in your document, before the <html> tag. It tells the browser which HTML or XHTML specification the document uses. Unfortunately, RadDock 1.x experiences some problems with the latest HTML specifications and this is the reason for the problem. We hope to have it fixed for v2 of the control. SOLUTION
For now you should not specify any DOCTYPE on the page.
Here are some resources on DOCTYPE usage: The global structure of an HTML document HTML Validation: Choosing a DOCTYPE