Okay I’ve done some more digging in the Discourse code base.
I found one reference to getRootNode, and it was added six days ago by @joffreyjaffeux (no personal blame intended by my post). The file is vendor/assets/javascripts/jquery.js (note that you may need to click to expand in the github diff as it’s a large file)
I believe this change has inadvertently affected backward compatibility with older browsers that we might hope that Discourse would continue to support (the user in my case was using an iPad Mini 2 running iOS 10.2, which is by no means prehistoric)