Created attachment 288252 [details]
Screenshot

Please see screenshoot for problem with gnome-panel.

https://bugzilla.gnome.org/show_bug.cgi?id=738158
This bug has screenshoot with same problem. When GtkMenu hits top panel it is adding scroll buttons when it should not do that. Bug itself is about different problem.

Looks like this is result after enabling csd for GtkMenu:
https://git.gnome.org/browse/gtk+/commit/gtk/gtkmenu.c?id=dae252e3d21c60e07076d2ab3ef81f9f99e19fe0

When menu is opened it has correct height in first calls in size_allocate and/or get_preferred_height. But then it has height -5px.

https://git.gnome.org/browse/gtk+/commit/gtk/gtkmenu.c?id=e9ed210b0be283676f54bcb232aebc66b9741825
This commit adjust y value. Looks like shadow is 5px and it is causing GtkMenu to get 5px smaller height.

In my example gnome-panel has 26px height. y for GtkMenu should be 26, but removing top shadow it is 21. That means 5px out of real screen space.

Same thing happens if gnome-panel is vertical and menu hits screen top/y- 0px. Then it will try to position menu -5px out of screen.

Any ideas how to fix this?

Maybe in case when window is at top of screen top shadow should not be added - border.top = 0;