The invention relates to the field of user interfaces for computing devices. The height adjusting mechanism is slidable between the supporting frame and the casing with a supported state of the casing by the supporting frame substantially maintained, and the supporting surface includes at least a first supporting surface set at a specified first height position and a second supporting surface set at a second height position different from the first height position. In the extended configuration an accessory may be attached to the displaceable post, whereas in the recessed configuration the displaceable post may be at least partially inaccessible. The powder is essentially dry and blowable powder.