I've been testing Sierra on my laptop since the public beta came out. As well as the time code window bug, sometimes the show/hide control on sections of the info panel will disappear so you can't access closed plugin panes and theirontrols, etc. Closing the info panel and opening it up again restores them most times but occasionally I have to restart Final Cut.
That's the only two bugs I've seen so far but I don't use my laptop for any serious editing so there may be more.
(For some reason I can't edit my own posts so apologies in advance for any stupid mistakes or bad English that I can't go back and fix)
Jeff Kirkland | Video Producer & Cinematographer
Melbourne, Australia | Twitter: @jeffkirkland