I *think* this is a theme issue, but I'm not sure. I've recently changed my site theme from More to Morecandy and everything works as before except the Progress Bar.
In courses with a lot of items tracked, it extends beyond the end of the box. I switched back and forth between the different themes and Morecandy was the only one where this happened. Switching editing on and off didn't have any effect. I checked Firefox and Chrome with the same results - the mobile browser had the same issue.
I'm using 2.9 and all plugins are up to date.
I'm not sure what else I can try, so am posting here in the hope of a tip.
Btw, this is the first time ever that I have changed the theme of the site and students have actually contacted me to say they really like the new look, so "Thanks, Mary!" (if you are reading this!)