Thank you, bug-submitter Marcin Kowalski for pointing me in the right direction! The newest version of GTK+ does not play so nice with MySQL Workbench (which has a habit of not being so new, and that is no fault of Gentoo’s). I’m ashamed to say, when Debian beats you to the punch with a patch, you’re not doing your job as a rolling-release distribution. That’s the slowest channel a patch could possibly go through.
/mnt/gentoo/var/tmp/portage/dev-db/mysql-workbench-6.3.4-r2/work/mysql-workbench-community-6.3.4-src/library/forms/gtk/src/lf_popover.cpp:399:13: error: no match for ‘operator!=’ (operand types are ‘Glib::RefPtr’ and ‘int’) if (dsp != 0) ^
If you’re trying to install MySQL Workbench and see something similar to the above, then apply the following user patch: