I eventually worked out how to restrict a ToolWindow to attach only onto a center dock guide when dragging.
Now I’m trying to programmatically attach a ToolWindow. Whenever I call the ToolWindow.Dock() it always docks to the side. I tried the overload specifying the IDockTarget and Side but this only allows a side to be set, setting side to null is not allowed. Have tried various property settings for CanAttach and CanDock on both DockSite and ToolWindow to no avail. Also set "e.Side = null" in WindowsDefaultLocationRequested with no success.
Selecting “Dock as Document” from the floating ToolWindow options menu has the desired effect but I need to be able to do it programmatically.