I use the Xfce4 desktop on Fedora 22. With the GTK3 Nightly builds, the tooltips (the floating textbox that appears when hovering the mouse over an element) have transparent backgrounds. Under Gnome, the tooltips have a solid pale yellow background. Other GTK3 apps such as GEdit and the various Gnome utilities running under Xfce4 do not exhibit the problem and have a solid yellow background for their tooltips.
Yes, sounds the same, thanks. Duping forward as the other has a patch, but please reopen if not fixed in the latest Nightly.