Yes, will try to bisect mesa. Unfortunately, in looks like I'll have to do that
manually, since Gentoo doesn't seem to have bisect tools for portage. So far I
can say following initial info:

1) Bug wasn't introduced at least until November 30, 2016.
2) White button artefact doesn't seem to be related to hang. In Nov 30 commit
button is white, but pressing it doesn't hang the system.
3) On Dec 20, 2016, hang was already introduced.