feat(web): session-list scrollbar tick for open session position

[heavygee]

Summary

• Adds a small visual-only tick on the session-list left scrollbar gutter showing where the currently open session sits in the scroll extent.
• Operators click the native scrollbar near the mark (not the mark itself) to jump; hover title explains the affordance.
• Hides when the open row is filtered out or lives in a collapsed group so the mark never lies.

Test plan

• Open a session in a long sidebar list; confirm a small tick appears on the left scrollbar rail at roughly that session's position
• Hover the tick; tooltip explains open-session position / click scrollbar nearby
• Scroll the list away from the open session; click the scrollbar near the tick; open session comes into view
• Collapse the open session's group (if possible without auto-expand fighting you) or filter it out; tick disappears
• cd web && bun run test src/components/SessionList.test.ts

Issues

Fixes #1732

[heavygee]

Proof (pr-attach-proof, release strategy — not user-attachments)

_{asset: heavygee/hapi@pr-attach-proof-1733/proof-2026-08-31T12-11-21-1732-session-list-open-tick.png · delete release after merge if desired}

[github-actions]

Findings

• [Major] Recompute after the collapse animation finishes — the effect runs when data-open changes, but the list panels animate grid-template-rows for 250 ms (web/src/index.css:359). Observing only the scroll container does not report changes to its scrollHeight, so the RAF at web/src/components/SessionList.tsx:1893 measures an intermediate layout. Collapsing or expanding any section above the selected row can therefore leave the marker at the wrong rail position until some unrelated mutation or window resize occurs.
  Suggested fix:

  resizeObserver?.observe(container)
  const content = container.firstElementChild
  if (content instanceof HTMLElement) {
      resizeObserver?.observe(content)
  }

Summary

• Review mode: initial
• One Major correctness issue: animated list-height changes can leave the open-session marker stale.

Testing

• Not run (automation). Add browser/component coverage that collapses and re-expands a non-selected section above the selected row, waits for the transition, and verifies the marker settles at the recomputed ratio.

HAPI Bot

[github-actions]

[MAJOR] Recompute after the collapse animation finishes

The panel state changes that retrigger this effect animate grid-template-rows for 250 ms (web/src/index.css:359). This observer watches only the scroll container's fixed border box, so it does not fire as the inner content changes scrollHeight; the initial RAF can capture an intermediate frame and leave the tick stale after collapsing/expanding a section above the selected row. Observe the inner content box (or handle transitionend) so the final geometry is measured.

Suggested fix:

resizeObserver?.observe(container)
const content = container.firstElementChild
if (content instanceof HTMLElement) {
    resizeObserver?.observe(content)
}