I'm committing this because it captures a repro for the situation
described across these two issues:
Refs: https://github.com/jekyll/jekyll-archives/issues/28
Refs: https://github.com/jekyll/jekyll/issues/6209
This commit has a workaround in place; to demonstrate the problem,
change this line of `_layouts/default.html`:
```
{% include toc.html sanitize=true baseurl=p.url html=p_content %}
```
to:
```
{% include toc.html sanitize=true baseurl=p.url html=p.content %}
```
Then navigate to various pages: the TOC in the LHS inconsistently
shows headers from different pages depending on the current page.
