GitHub glitch - merging master into an old pull request

[John Neffenger]

I was surprised to see GitHub listing in my pull request all of the 
changes I merged from the upstream master branch, as I commented here:

Did I merge the upstream master branch correctly?
https://github.com/openjdk/jfx/pull/60#issuecomment-614885532

Unfortunately, GitHub also pulled into my pull request anyone who 
participated in those changes to master, which is why I'm posting this 
to the mailing list instead of commenting there.

This is the first time I've seen the problem, but it dates back to at 
least 2013 according to this Stack Overflow question:

GitHub pull request showing commits that are already in target branch
https://stackoverflow.com/q/16306012

Should I try the suggested fix of temporarily switching the base branch?

In my case, I would edit the pull request to choose the "jfx14" base 
branch, save the change, and then edit it again to switch it back to the 
"master" base branch. The Stack Overflow answers state that the extra 
commits will then be gone, and I'll have a nice clean pull request again.

Thanks,
John