Comment 10 for bug 1407880

.window-frame {
    box-shadow: none;
   }

Should be enough, I switched to using exactly that to solve the same problem with my hacked mate menus and gtk recognizes it just fine. The border-width has nothing to to with this, it is entirely the shadows. I didn't keep cairo-dock installed after today's tests but I see no reason this should not work the same for cairo-dock as it does for any other window. The key is to use it in the cairo-dock theme (or in my case panel theme), not the global gtk3 theme. This way the fix works no matter what gtk theme a user might select, and does not affect shadows on things like application menus.