This is a serious problem when using Glade. Upstream report: https://bugzilla.gnome.org/show_bug.cgi?id=763624 Patch that fixes it committed to upstream master: https://git.gnome.org/browse/glade/commit/?id=ac55fa78566145ac44d48ba88ec1351db5b4a99d
Fixed by dev-util/glade-3.20.2 now in the tree.
Thanks for the feedback and sorry for the long delay.