[css-overflow-5] ::scroll-marker should determine inherited interactivity from ::scroll-marker-group

[flackr]

It's common practice to make the offscreen pages in a carousel inert, however, the ::scroll-marker pseudo-elements generated for them should not be. In general, it would make sense for them to inherit their interactivity from the ::scroll-marker-group as this is the control into which they are placed.

E.g. in the following example, only the current page is interactive, but the markers to go to the other pages are expected to be interactive.

<style>
.scroller {
  scroll-marker-group: after;
  overflow: auto;
  scroll-snap-type: x mandatory;
}
.page {
  interactivity: inert;
  @container scroll-state(snapped: inline) {
    interactivity: auto;
  }
}
.page::scroll-marker {
  counter-increment: page;
  content: counter(page);
}
</style>
<div class=scroller>
  <div class=page></div>
  <div class=page></div>
  <div class=page></div>
  <div class=page></div>
</div>

[flackr]

If actually inheriting from the scroll marker group is difficult, we could disallow interactivity on ::scroll-marker and have its used interactivity be that of the containing ::scroll-marker-group. This would mean that authors couldn't individually control the interactivity of the scroll markers, only the entire group, which doesn't seem like there's much of a need for. I suspect we could revisit this.