I did a few more tests. I have a test version of the site (running with older site data) on a different computer and it does not exhibit this problem for the user. Since the site configuration is pretty much identical it is surprising - there are more posts and more users etc but the underlying configuration has not changed.
The test version of the site is on a computer running on an older Ubuntu OS, and its an actual machine as opposed to a virtual one. I run a proxy on the actual site but when the user detoured the proxy she still had the problem so its not the proxy. Well, the live Discourse is on beta 10 and the test is beta 11, I doubt that matters but I could check that out. I could also restore a backup of the live site on the test and see if that breaks the test site.