Are windows just ordinary widgets?

MacOS treats windows and "controls" (i.e. widgets inside a window) as 
completely different things. Most other platforms treat them as 
essentially the same things. Which route should the CGA go?

The situation seems to be something like this:

+ Widget
  everything that occupies space within some coordinate system.
  Has attributes like size.
	- Window
	  A toplevel Window. There will be several variants of this, for 
document windows, dialog boxes etc.
	+ Pane
	  A widget "inside" a window (a window has panes -
	  I've seen this name somewhere, feel free to suggest a more standard 
one)
		- Button
		- GridBox
		- Label
		- etc.

"Container" could be a class outside this hierarchy, implemented by 
"Window", "GridBox" and a few other widgets.
A Container would only contain Panes, never other widgets.
The relationship between a window and a window-modal dialog window has 
nothing to do with the relationship between a window and a button 
inside that window, IMHO, so they shouldn't be expressed using the same 
parent/child relationship.

Now perhaps we want a common datatype "Widget" with functions
     toWidget :: Widget w => w -> Widget
and fromWidget :: Widget w => Widget -> Maybe w

Perhaps we want a datatype "Pane" (or "Control" or whatever) with
	toPane :: Pane p => p -> Pane
and so on.

If I had a "Pane" datatype, I wouldn't really need the "Widget" 
anymore, but of course, one can never know.
The "Pane" datatype would be the natural place to implement a lot of 
functionality at least on Mac OS, so if it's there, I'm happy, and I 
could then implement a Widget datatype as a simple discriminated union 
datatype. If there was no Pane datatype, the implementation for the 
widget datatype would become slightly ugly and inelegant on Mac OS...


Cheers,

Wolfgang