Split Horizontal & Vertical Visibility Values

I would like to see visibility split into two properties: 
'horizontal-visibility' and 'vertical-visibility' (with the possibility of 
leaving in the current property as a shortcut).  With these, an author can 
make an element not visible, but still have a presence "felt" in the layout 
in a particular direction.

A good use of this is a tree control (like those on ms.com's sidebar) whose 
width is guaranteed not to change as branches are dynamically hidden and 
shown (therefore avoiding nasty reformatting artifacts/flickers).

Andrew n marshall
  student - artist - programmer
    http://www.media-electronica.com/anm-bin/anm
      "Everyone a mentor,  Everyone a pupil"