[css-overflow-5] Creating scroll-marker groups within which to select an active marker

[flackr]

In order to be able to select one active scroll marker among many potential options, we need a way to establish a group of scroll markers.

In https://flackr.github.io/carousel/examples/scroll-marker/scrolltarget/ there are situations in which more than one group is desired (TOC section selector on left is one group, chapter selector icons on right is another group), as well as where some anchor links should not be part of the group (e.g. the inline links to other sections within the document should not be made active.

The current draft specification establishes a group by having an ancestor with a specified focusgroup value, however there are other options. Should we:

1. Continue to use focusgroup as the mechanism for creating a scroll marker group. It seems to have the correct semantics, though there we may want an alternative as:

   • it does mean that anytime you establish a group you will also get the focus-group up/down keyboard navigation behavior, and
   • as I understand it, focusgroup is not yet in a spec: https://open-ui.org/components/focusgroup.explainer/

2. Add a named mechanism. If so, we'd have to decide whether this is:

   • An HTML attribute? E.g. similar to radio group's name attribute, however name already has a defined meaning on links so we would need to choose something else, maybe group?
   • A CSS property?

   And whether this is a property on each participating link (similar to radio buttons) or on an ancestor (similar to focusgroup).

These options are not necessarily mutually exclusive. We could implicitly create marker groups from focusgroup, but also allow explicitly putting them in a named group.