🚀 go-pugleaf

Take a look at amulet (at http://www.cs.cmu.edu/~amulet) it's a free GUI
toolkit for cross platform development (X11, windoze and mac). It's
written in C++ on top of Xlib (in the unix flavour).

I think it's got a handful of very nice concepts/ideas built in:
 
*contraints*: most objects' attributes can accept functions in addition
to values in order to restrict/control the allowed values. for instance
you can use a function to set the width of a graphical object to be a
function of another object's width/height/whatever. Net result: writing
geometry managers is a no-brainer.   
 
*interactors*: you have built in mechanisms to deal with most common
user interaction paradigms: single one shot events, creating new
objects, moving objects, etc. The amulet framework already supports many
nice features you can control like rubber-banding, gridding (when moving
objects), undoing, selecting (grouping/ungrouping),... Net result: easy
to write an advanced graphical editor.

It's really much, much more than a widget set. You can argue it has a
rather strange design, doesn't use C++ effectively (STL, etc), doesn't
use Xt resource manager but anyway i think the pros greatly overtake the
cons. 


J Cameijo Cerdeira