Whenever I launch GTK+ applications over x2go, the pull-down menus are usually not displayed properly or at all.  The server I'm connecting to is running GTK+ 3.22.3; the local client is running GTK+ 3.14.15.

For example, if I launch the Xfce Terminal, I see the usual "File", "Edit", "View", "Terminal", "Tabs", and "Help" labels in the pull-down menu bar.  If I click on these, they become underlined as though they are active, but the corresponding pull-down menu doesn't (usually) appear.  Instead, a single pixel in the upper left corner of my desktop turns white.  The only exception is when I click on "Help" -- the first time I do this, I do get a pull-down menu, but it's attached to the upper left corner of my desktop, not to the "Help" label in the Terminal window. The menu is very small, though, and I need to scroll through it to see all the entries.  The second or subquent times I try to view the "Help" menu, I just get a single pixel.

Similar behaviour is observed in most other GTK+2 and GTK+3 applications, such as the GNOME Terminal and the GNOME Character Map. The problem does not occur with any non-GTK+ applications.  Some (but not all) of the applications output error messages like this to the console when I try to activate a pull-down menu:

(gucharmap:26208): Gtk-CRITICAL **: gtk_widget_get_preferred_height_for_width: assertion 'width >= 0' failed

(gucharmap:26208): Gtk-CRITICAL **: gtk_widget_get_preferred_height_for_width: assertion 'width >= 0' failed

(gucharmap:26208): Gtk-CRITICAL **: gtk_widget_get_preferred_height_for_width: assertion 'width >= 0' failed

(gucharmap:26208): Gtk-WARNING **: Negative content width -7 (allocation 1, extents 4x4) while allocating gadget (node arrow, owner GtkMenu)

(gucharmap:26208): Gtk-WARNING **: Negative content width -7 (allocation 1, extents 4x4) while allocating gadget (node arrow, owner GtkMenu)

Claws Mail seems to be an exception; it uses GTK+ but its pull-down
menus work fine.

This has been discussed on the x2go users mailing list at <http://lists.x2go.org/pipermail/x2go-user/2016-November/004068.html>.  Another user has the same symptoms and reported that downgrading to GTK+ 3.20.9 fixed the problem.

So far I can reproduce the problem only when running GTK+ applications over x2go -- maybe other NX-based remote execution frameworks are affected, though I haven't tested any.  The problem does not occur when running GTK+ applications locally on the server or client, nor when running applications remotely over a vanilla ssh connection with X forwarding.