Hopefully we can put this one to bed but I thought I'd chime in with experience from using Sierra (10.12) for a few days.
- The tab closing/switching issue is still gone, hallelujah, and I've had no trouble yet from closing opening any tabs containing videos playing.
- I've yet to have the same stuttering freeze that the last version of Safari gave me (I mentioned it in my last comment).
- The Vimeo graphics glitch I've had the entire time appears to have also disappeared (occasionally vimeo HTML5 embeds would show a glitched graphic effect instead of their placeholder image until I scrolled away and returned).
- It appears to have solved by issue of Safari frequently causing an isolated tab to freeze up and choke the CPU until I closed that tab.
Unrelated to the above or this problem but it also fixed that horrible wifi disconnect problem I (and others) were having where the laptop would show a stable wifi connection and accessible internet but would drop traffic completely for about 30 seconds every 15 minutes or so (while no other connected devices did). I actually rushed updating to 10.12 in the hope it would address this as I didn't want to fork out for a new router which appeared to be the only other solution.
I'm having very few programs refusing to run in 10.12. I had a lot more going from Yosemite to El Capitan.
Lastly, a thanks again to any Apple engineers reading this who helped with this issue on the quiet. While it's frustrating that these (major) problems are not acknowledged, I'm glad there's still people reading these reports and ultimately issuing a fix.
