Checker interface covers Back To Top module widget

Indeed.

I think the best thing here because of the late fade-in is to use CSS to move the back to top widget instead, so you don't get a weird floatie-thing going where Editoria11y is sorta vaguely centered before the other widget appears. Something like:

body:has(ed11y-element-panel) #backtotop {
  margin-right: 140px;
}

I'm a little loathe to add that sort of thing in my module, since I'd be moving their widget; might be good to ask Back to Top to add an offset parameter?