Comment 8 for bug 1369971

My father was having this problem on Ubuntu 14.04 LTS. The problem began several months ago and was intermittent. When he tried to shutdown his computer, the shutdown dialog would appear, but none of the buttons would be responsive. Even the X button to close the dialog would not be clickable. Instead, the mouse would interact with whatever window is in the background. E.g. if firefox was open, he could click on the menu, scroll the window, etc... all while the shutdown dialog window remains in front, blocking the view. This suggests that the problem is that the dialog windows appear, but never or can't acquire focus.

My father worked around this by pulling the power on his computer... eventually ruining his hard drive. In replacing his hard drive, I decided to do a clean install of Ubuntu 16.04 LTS on his computer. Immediately after the install, his computer would shutdown fine. Though less than 24 hours later, this same problem was popping up and he couldn't shutdown his computer through the usual shutdown dialog window. However, this time he left the computer alone... when he came back, the lockscreen had appeared. Upon entering his password to dismiss the lockscreen, the shutdown dialog was able to acquire focus and he could interact with it! He is currently working around this issue by launching the lockscreen whenever he can't interact with the shutdown dialog window. It is a huge annoyance, but it works.

It is worth noting that my father uses only 2 applications on his computer, firefox and skype. Skype runs automatically at boot. Firefox is the first and only thing he launches after login.

Hope this sheds some light on the problem and leads to a fix.