On Lubuntu 14.04 x86 default clean install, pressing Alt+Tab results in UI error.
Steps to reproduce:
1. Open Writer or Calc and select File to activate File sub-menu
2. do not click or otherwise deactivate the active sub-menu
3. press Alt+Tab
Sub-menu stays in focus although the background application has changed
Focus is entirely transfered from LibreOffice to another application
Operating System: Ubuntu
I could reproduce this is xubuntu 14.04 (but not ubuntu 14.04) but it was the same behaviour when doing it from within Thunar, so its likely a gtk issue.