as per https://bugs.launchpad.net/ubuntu/+source/gtk+2.0/+bug/749986/comments/4 this has been introduced by backporting patches from gtk3 to gtk2 in Ubuntu packaging. gtk3 is unsupported/early-prototype for LibreOffice, so backporting these patches to gtk2 is the rootcause for this error (and why this does not happen on other distros).
as per https:/ /bugs.launchpad .net/ubuntu/ +source/ gtk+2.0/ +bug/749986/ comments/ 4 this has been introduced by backporting patches from gtk3 to gtk2 in Ubuntu packaging. gtk3 is unsupported/ early-prototype for LibreOffice, so backporting these patches to gtk2 is the rootcause for this error (and why this does not happen on other distros).