Started working on it in late 2016, as a separate library for GTK 3, to be used by Endless for their applications. After writing the layout manager delegate class, I've spent the last 3 months cleaning up the solver and the various moving pieces; with the help of Matthias Clasen, we finally got it merged today, with a lot more features.
Of course, constraint layouts have a cost. Specialised, finely tuned layout managers beat general purpose ones. If you're shoving 100s of rows in a list box, you probably want to keep using the GtkListBox widget; but if you have a form view, or a complex UI with a known set of widgets, do give constraints a try, and you can avoid lots of nested boxes and messing around with alignment and expansion flags
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