When viewing the table of contents in a book activity, the block appears below the text but still on the right side of the screen when viewed in Internet Explorer 11 on a desktop:

This displays properly in Chrome, Firefox and Edge.

Reproducing the issue

1. Login as a Moodle administrator.
2. Ensure that the Boost theme is active (not an issue in Classic)
3. Create a new course.
4. Add a book to the course
5. Add a chapter to the book. Just a few words in the chapter titles and content fields are sufficient.
6. Save

Look for the automatically generated table of contents. You will find it in the bottom right area of the page.

Note: It did not seem to affect themes based on the Boost theme nor does it affect the Classic theme.

Why is this an Accessibility issue?

Because expected content is in an unexpected location making it hard to locate for users of screen magnifiers. It also affects others if the content is long enough to push the table of contents block below the fold.

What I expected

I expected the table of contents to appear to the right of the contents, as it does in all other web browsers.