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Currently RadDock does not change its position when setting its x-coordinate via the function set_left()
changes only a value in the RadDock object but does not change the x-coordinate of the corresponding html element which represents the dock object. To achieve the desired functionality I suggest you the following workaround which manipulates the html object, e.g:
$get("RadDock1").style.left = "400px";
is the id of the RadDock object.
Please, let us know if you need further assistance.
the Telerik team