Please see RT-21772 for a test case. Also, please make sure your FX build contains a fix for this issue (it's just been pushed to the Graphics scrum).

1. Compile and run the test attached to that issue.
2. Click the full screen toggle button in the window title bar
3. Open a menu.

You will see that the menu is positioned far below from where it is expected to be positioned. I've added debugging output to Glass and seen that such bizarre position is actually requested by FX code (please see the comments for RT-21772 for some tips). Looks like the menu code takes into account the screen and/or window insets even though the window is in the full screen mode when the menu is being opened (and all the insets are obviously zeros at this time).