- but I managed it... (this is not your fault, but my bad english..)
If the designer would let you set this property the user-control would be docked in the design view. I understand why this property was hidden (anyway why should a control be "docked" by design? - this should always be controlled by the container in my opinion.)
What I'm wondering is why you had difficulties to implement this feature (you are well known here on cp - and I think you are a real expert )
There is also another possibility (but it works only for containers that not contain any other controls)- use the Docking attribute for your UserControl: