How to enable overlay scrollbars in my app?
You should use gtk_scrolled_window widget because is the only widget that support overlay scrollbars.
Source https://wiki.ubuntu.com/Ayatana/ScrollBars
If it doesn't work try with:
sudo su
echo "export LIBOVERLAY_SCROLLBAR=1" > /etc/X11/Xsession.d/80overlayscrollbars