This is a Gtk bug (feature?) as I just observed the same issue (on same machine where Inkscape has this problem) in another Gtk application (LAG by arsf).
I see the same issue in Inkscape 0.91 running Ubuntu 16.04 (XFCE), but 0.92 on my Gentoo ~AMD64 (KDE) seems to not be affected.
This is a Gtk bug (feature?) as I just observed the same issue (on same machine where Inkscape has this problem) in another Gtk application (LAG by arsf).
I see the same issue in Inkscape 0.91 running Ubuntu 16.04 (XFCE), but 0.92 on my Gentoo ~AMD64 (KDE) seems to not be affected.