This short post is to inform all Debian MATE users that the recent GTK-3+ upload to Debian (GTK-3+ v3.21) broke most parts of the MATE 1.14 desktop environment as currently available in Debian testing (aka stretch). This raises some questions here on the MATE maintainers' side...
Isn't GTK-3+ a shared library? This one was rhetorical... Yes, it is.
One that breaks other application with every point release? Well, unfortunately, as experience over the past years has shown: Yes, this has happened several times, so far — and it happened again.
Why is it that GTK-3+ uploads appear in Debian without going through a proper transition? This question is not rhetorical. If someone has an answer, please enlighten me.
Potential Counter Measures
For Debian MATE users running on Debian testing: This is untested, but it is quite likely that your MATE desktop environment will work again, once you have reverted your GTK-3+ library back to v3.20. For obtaining old Debian package versions, please visit the https://snapshots.debian.org site.
The MATE 1.16 release is expected for Sep 20th, 2016. We will do our best to provide MATE 1.16 in Debian before this month is over. MATE 1.16 will again run smoothly (so I heard) on GTK-3+ 3.21.
sunweaver (who is already scared of the 3.22 GTK+ release, luckily the last development release of the GTK+ 3-series)