Seems DiscoTOC - automatic table of contents has become buggy recently.
Depending on viewing full screen or reduced screen in Chrome, see screenshots, the Contents and Timeline buttons will float from immediately under the timeline where I have always seen them to the bottom of the screen making the but unnoticeable.
Full screen
Reduced screen
There seems to be inconsistencies using Firefox as well.
Here is a collection of topics with issues. Editor Issues
Edit: reproducible here on Meta using Chrome and Firefox. With FF the button never moves off the bottom of the page.