Katex block overflow

[JanDW]

• set overflow-x: scroll;
• make sure it is keyboard focusable tabindex=0, but only when it actually overflows: scrollWidth exceeds clientWidth for the container
• provide signifier to indicate overflow is present

Work is done is katex-overflow branch.

[JanDW]

Also, when the viewport is wide enough, should the block just take up more available space and break out of the container?

[JanDW]

Improve accessibility:

• [ ] https://www.a11yproject.com/posts/2021-01-28-how-to-use-the-tabindex-attribute/#toc_Scrollable-overflow-containers
• [ ] See table component in Heydon Pickerings Inclusive Components book