H client doesn't work properly with NYT comments

From Zendesk ticket: https://hypothesis.zendesk.com/agent/tickets/1996

## Feature Request Form
### Problem you are trying to address with this feature
When annotating comments on the New York Times website, the H client doesn't work as expected. If you highlight text within a comment, the "highlight" and "annotate" prompts don't appear (though you can still manually highlight/annotate in the sidebar). Sharing a link to an annotation of a comment just leads to the article itself rather than the annotation. Also, when you look at annotations on comments, the h sidebar covers the comment itself.

### Steps to reproduce
1. Go to this NYT article: https://www.nytimes.com/2018/01/16/magazine/beyond-the-bitcoin-bubble.html and activate h
2. Click on the comments button at the top of the screen (next to the "Share" button"). A sidebar with comments will pop over the web page
3. Highlight text within one of those comments. Notice the "highlight" and "annotate" prompts don't appear below the highlighted text. Also, the H sidebar shows the "annotate" button but not the "highlight" one.
4. View a public annotation I have made on one of the comments: https://hyp.is/1r2elALEEeiG28uaU16SZg/www.nytimes.com/2018/01/16/magazine/beyond-the-bitcoin-bubble.html - Notice this just takes you to the article, not to the annotation itself

### Browser/system information
User was on Chromium Version 63.0.3239.132 (Official Build) Built on Ubuntu ,
running on Ubuntu 16.04 (64-bit). I reproduced this using the latest version of Chrome for MacOS.

### Additional details
I see this as related to but slightly different from https://github.com/hypothesis/product-backlog/issues/25. I believe this is happening because the NYT shows their comments as a sidebar that pops over the page. I don't think the NYT is the only site that does this, so if I find other examples I'll update here.