No worries. When I looked before it didn’t look like the KaTeX people had any especially good ideas for making KaTeX-rendered display math more mobile-friendly.
Personally I don’t much like the horizontal scrollbar solution on websites where I have seen it used, but it is better than just overflowing off the right side with no ability to see chopped-off content at all.
Personally my preference on mobile for equations is for them to be just shrunk so that they fit on the page, but with the mobile browsers’ ability to zoom not disabled, so any equation that is illegibly small can be expanded.
I added a function for this @jrus/misc#responsive_katex which can be seen in use at https://observablehq.com/d/fb4d519e95cdbd83
It might not be easy to do this in an automatic way that doesn’t break anything else / stomp on other authors’ preferences though.