I've also seen these errors, except that they're showing up in my tmux panes, which is significantly more annoying than .xsession-errors. This is probably because vim-gtk3 (gvim) links to libfontconfig1.
libfontconfig definitely shouldn't be producing errors to stdout or stderr unless specifically requested by the calling application. -- brian m. carlson: Houston, Texas, US OpenPGP: https://keybase.io/bk2204
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