In my Arch Linux installation I have found that since I upgraded from GNOME 3.22.x to GNOME 3.24.2 that in the power off dialog one is now needed to click twice on any of the buttons in order to make them do anything ("Cancel", "Restart" and "Shutdown" buttons). Or rather, one is needed to click anywhere on the screen before the buttons respond, it is up to you, you can click twice on the button you wish to do something, or you may click anywhere else including on any of the other buttons, then just click for the second time on the button you want to and it will realise that you have clicked it. The first click doesn't even trigger the little animation of the button.

I said that all of the buttons have this issue, but actually that only started today, previously the "Cancel" button would work with just one click, but after days of that button working, it has now stopped behaving as it should, and started behaving like the others. However my friend with Ubuntu GNOME 17.04 and GNOME 3.24.x tells me that his "Cancel" button still works as expected while the others do not.

As this issue is also present with Ubuntu GNOME I have also filled a report downstream there as LP reports are listed in search results whereas GNOME Bugzilla ones are not: https://bugs.launchpad.net/ubuntu/+source/gnome-shell/+bug/1690558