Since Gnome 3.26, gnome-session dialogs (Power Off, Logout, Restart) more often than not lose keyboard accessibility. The only way to get out the dialog is using a mouse.

Happens for me on fully updated Antergos linux. First time such dialog is called (or early in the session), keyboard accessibility works ((too) tiny blue line highlighting default button, changeable with tab key or arrow keys). Every next time (or perhaps first time later in the session), the dialogs appear without such blue highlight, and the dialog cannot be controlled using the keyboard.

Steps to reproduce: logout or shut down and select "cancel". Likely from the second time onwards, there will be no keyboard accessibility anymore.

May be related with 775869 ("Logout" shortcut causes Power Off dialog to open twice and lose keyboard accessibility) but not the same: it is probably not a matter of two dialogs appearing, and selecting a button works with a single mouse click.