I had the same issue: After updating to WP 4.3 my text editor tab “button” stopped working.
My fix was disabling all plugins and reactivating one by one. First indication it was a plugin: after deactivating them all the button worked. I found one plugin that hadn’t updated in over 4 months so I switched to an alternate plugin and corrected the issue.
Just thought I’d mention that I also tried changing to different default themes but that obviously wasn’t the issue for me. It’s worth trying if you are having this issue though.
