I have found why my phone behaved so weirdly.

The maintainer of libgtk2.0-0 package forgot (or didn't know) that one must do update-gdk-pixbuf-loaders after package installation. What's even more strange and weird is that /etc/gtk-2.0/gdk-pixbuf.loaders was overwritten at all. Why in the world would one want THAT?

Okay. I did update-gdk-pixbuf-loaders, and all is good and sweet except that the on-screen keyboard is gone. So is Fn+Sym.

How do you guys test your updates? Do you ever try incremental upgrades from PR1.3 to the last one? Do you do that on real devices? Do you test if custom settings are retained?