Unable to select new branch in PR

I have recently started having an issue where I cannot actually select a new branch from the dropdown that appears when I edit the title of a PR.  I can open the dropdown, and I see all the branches available in the repo.  However, when I actually click on a branch name nothing happens.  I have tried clearing all site data and restarting my browser, but that did not change anything.  There are no errors on the browser console when I try this, though there are three Content Security Policy errors that occur when I load the page (or any other page on GitHub).  I’m at a loss as to what could be causing this.

Browser: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:61.0) Gecko/20100101 Firefox/61.0

I have done some more experimenting.  I can change branches if I use Chrome instead of Firefox.  It appears that the pop-up warning about changing branches is opening and instantly closing on Firefox.  I have all pop-up-blocking extensions disabled for GitHub, and I also tried disabling pop-up blocking globally in Firefox.  Neither solved the issue.

 Did some testing in Firefox Developer Edition, and I was able to select branches.

Not sure what I can do to help, but

I can suggest that you should switch to Chrome. You don’t have to, but just suggesting. :wink:

Hi @vonmoltke,

Thanks for opening this. I just tried this myself and was unable to reproduce the problem. Are you still seeing this issue occur in Firefox?

Cheers!

I confirm that on my old machine, I cannot select a branch on github, the dropdown menu is simply not visible. I am on a very old OS, so it might be normal, but just wanted to let you know.

Windows XP SP3 Win32  / Tried both  Firefox 52.9.0 and Opera 36.0

It happens when for example I have the following just under the supposed dropdown in the PR:

"There isn’t anything to compare. "

 Hope it helps

Carl