I noticed that during a FMS update the 100% icon was displayed for over 5 minutes. So I opened the $FGLHOME/state/splash/foglightStartup.html file which showed that cartridges were still being enabled.

Then after the carts completed, it continued with the Remote Services and when that was complete, finally the Install Wizard completed and the splash screen showed the URL to login.
The UI progress meter goes all the way to the right based on activity, not on progress.