Yeah, I don't understand the grandparent on that. The Xt API (and the Athena widget set) were hardly a great work of computer science. But they were relatively clean and easy to understand, and basically invented the "widget" (rectangle on the screen that can draw itself and respond to input in an encapsulated way) component model that we're all still thinking in today.
I wouldn't say Xt invented the "widget", and I would never pass up the opportunity to state most emphatically that Xt widgets are neither clean nor easy to understand.
Here's a video tape by Brad Myers called "All the Widgets" [1] that shows demos of widgets from way back, many of them long before Xt. (Over 175 segments, from 30 systems, from 15 companies!)
All the Widgets, by Brad Myers. This was made in 1990, sponsored by the ACM CHI 1990 conference, to tell the history of widgets up until then. Previously published as: Brad A. Myers. All the Widgets. 2 hour, 15 min videotape. Technical Video Program of the SIGCHI'90 conference, Seattle, WA. April 1-4, 1990. SIGGRAPH Video Review, Issue 57. ISBN 0-89791-930-0.
What kicked Xt's and Motif's ass, especially in terms of power and flexibility, as well as being clean and easy to understand, was TCL/Tk.
The reason TCL/Tk was so successful in spite of how lame TCL was as an scripting language, is that TCL was there from day one of Tk's design, not an afterthought.
As a result, there was no need for Tk to invent a bunch of half-assed kludges, and require applications to build complex Rube Goldberg devices on top of those, when Tk could simply call into TCL to do anything the scripting language interpreter could handle, including calling into the application's C code from TCL.
So there was zero overlap between what the scripting language could do, and what the toolkit needed to do. And that made the toolkit vastly simpler and more consistent, and much easier to flexibly program and modify.
Xt based toolkits like Motif [2] had to built on top of all that half assed pseudo-object-oriented crap trying to reimplement programming language level concepts like objects, components, properties, defaults, inheritance, events, handlers, delegation, layout, etc, in brittle C and macros.
Motif was yet another proof of Greenspun's tenth rule of programming [3]: Any sufficiently complicated C or Fortran program contains an ad-hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp.
