Preserve scroll position on LG->Cohort Members

[michaelchadwick]

Fixes ilios/ilios#7305

Lots of time toying with complicated modifiers and scroll event listeners, but finally got a simple one to do the trick that uses the scrollend event so that it only triggers when scrolling stops, not on every scroll event. It only works with modern (Dec 2025 or newer) versions of browsers, but will just be ignored otherwise.

Note: there already exists a PreserveScroll service, but that works when transitioning between routes, and this PR adds an element modifier that can remember scroll position within a route.

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[michaelchadwick]

NTS: Might be able to fix with different way of tracking the <tbody> vs capturing scroll position.

[jrjohnson]

I think the answer is in packages/frontend/app/components/learner-group/root.gjs

Somewhere in...

  @cached
  get usersForCohortManagerData() {
    // Learnergroup members are only referenced here to trigger a re-computation on membership changes.
    return new TrackedAsyncData(
      this.getUsersToPassToCohortManager(this.args.learnerGroup, this.args.learnerGroup.users),
    );
  }

  get usersForCohortManager() {
    return this.usersForCohortManagerData.isResolved ? this.usersForCohortManagerData.value : [];
  }

  async getUsersToPassToCohortManager(learnerGroup) {
    const cohort = await learnerGroup.cohort;
    const topLevelGroup = await learnerGroup.getTopLevelGroup();
    const currentUsers = await topLevelGroup.getAllDescendantUsers();
    const users = await cohort.users;
    return users.filter((user) => !currentUsers.includes(user));
  }

...there is a place we could cache this data so we didn't return the [] empty during the rebuild.

My suspicion is that it's in:

const topLevelGroup = await learnerGroup.getTopLevelGroup();
    const currentUsers = await topLevelGroup.getAllDescendantUsers();

...but might be somewhere else.

What should happen when a user is moved is that the array decreases by one person and, that will trigger a re-render in the shadow dom, and that one row will get removed.

What is actually happening is that @users.length in cohort-user-manager.gjs is getting set to [] for just a moment. This causes the entire list to get destroyed and recreated.

This is worth tracking down, the scroll position tracking feels like a bandaid to me.