... Gosh, something odd just happened with this. I submitted the comment, and what appeared to be an empty comment from me appeared in the thread. Editing it brought back the content; submitting it again gave an empty-looking comment again. After a couple of iterations of this it looked normal. I don't know whether if I'd just waited it would have finished loading something and recovered.
[EDITED to add:] Same happened with this comment. This time I tried just waiting, and after about five seconds the content appeared. This is a really bad user experience; if things are going to be loaded asynchronously, it's probably better to have some indication that it's happening.
[EDITED again to add:] On submitting the edit above, initially the old version of the comment was displayed, and then after some seconds the edited version appeared. Again, not a good user experience.
I thought that instead of everyone having to create a separate post for their first impressions, it would be more convenient to create a single post for this discussion. I'll post my own here soon.