Show/hide the TOC based on a button. This doesn't animate, though it could if we wanted.
Fixes a bug with the links in the TOC, which @cath9 thinks "should actually work." Ugh fine now they do.
Is more explicit about the desired behavior of scrollbars. Because this layout is familiar to our users from Google Docs and elsewhere, it hides the intra-div scrollbars on browsers that might otherwise show them. We can change this behavior if users find it confusing.
Is more explicit about how the dividing line between the sections is drawn. Before it was something of a side effect that the active TOC area was not offset with a line. Thanks @matteocargnelutti for looking at that with me.
Couple features/extensions to reading mode:
TOC open
TOC closed