I am running Xfce4.10 on Fedora 21 with GTK3 3.14.8. Whenever GTK3 has a dark theme set, all GTK3 tooltips briefly glitch and flicker an image of the top left corner of the screen, then show their contents normally. This issue does not occur in Gnome Shell or in Openbox. Enabling/disabling the compositor has no effect. It occurs both on my x86_64 laptop (with the free radeon drivers) and also on my Samsung ARM Chromebook (with fbdev driver).
I reckon this is a bug in gtk, nothing to do with xfwm4, the same occurs under Waylamd
tooltips are override redirect windows, not under the WM control.