Yesterday evening when I booted my laptop and logged in, gnome started, but things were a bit strange. I could launch applications, actually a single application, e.g. Firefox, but it would start without any window handles (minimize, maximize and close) and missing the title bar. Moreover, I could not switch to another application if it was already launched.
So, after a quick search found out that it is a documented bug in Fiesty where "metacity" fails to launch. Since metacity is the default window manager, and it wasn't being launched after startup/login. All I had to do was to launch "metacity" from command line and things got back to normal. It also was able to run metacity the next time I logged in after logout and restart.
As always with anything Ubuntu related, I am not the first one to face this and the forums provide excellent information: