Back to top/menu jump (not scroll) on Desktop Chromium browsers.

Here’s the official page for a [theme](https://www.indocreativemedia.com/wordpress-theme-demo/simple-flat/)
On mobile, the back to top and menu scroll as expected.
On desktop (for me) in Edge, Chrome, and Opera, it just jumps without scrolling.
In FF it works OK.
Is this a Javascript issue. I’d love to know how to fix it. I don’t want to just use a back to top WP plugin, I’d like to fix or completely change the code that the theme is using. (I’m using a child of the theme.)
Thanks for any advice. I’m used to tinkering with HTML and CSS, but not Javascript at this point.

Also in #JavaScript _↴