Keep Grid scroll position when opening a task. - #70536

Summary

In the Dag details view, the Grid on the left keeps its own vertical scroll position. Clicking a task that is scrolled below the fold currently snaps the Grid back to the top the first time you do it — losing your place. This PR keeps the scroll position across that navigation, while still resetting to the top when you leave the Dag entirely and come back.

Problem

On a Dag with enough tasks to overflow the Grid, scroll down and click a task that is off the top of the viewport. The list jumps back to scrollTop = 0 (the first task) even though the layout looks unchanged. Clicking a second task (task → task) does not reset the scroll — it stays put. So the reset happens exactly once, on the first click, and it gets more painful the longer the list is (e.g. when task groups are expanded).

Screenrecords

Original

Current

Root cause

The Grid lives inside DetailsLayout, and every top-level page renders its own DetailsLayout:

• dags/:dagId → <Dag>
• dags/:dagId/tasks/:taskId → <Task>
• dags/:dagId/runs/:runId → <Run>
• dags/:dagId/runs/:runId/tasks/:taskId → <TaskInstance>
• <GroupTaskInstance>, <MappedTaskInstance>

These are sibling routes in router.tsx, not nested under a shared layout.

• First click (<Dag> → <Task>): the matched route element changes, so React unmounts <Dag> (destroying its DetailsLayout, its Grid, and the scroll container DOM node) and mounts a fresh <Task>. The new scroll container starts at scrollTop = 0, and useVirtualizer reads 0 on mount and renders the top rows.
• Subsequent clicks (<Task> → <Task>): the matched element type is unchanged; only the :taskId param updates. React keeps the instance and its DOM, so the scroll position survives.

That asymmetry — cross-element on the first hop, same-element afterwards — is why it resets exactly once. It is not task-group specific; groups just make the list longer so more scroll is lost.

Solution (this PR)

Remember the Grid's scrollTop across the remount and restore it, scoped to a single Dag visit.

• Persist + restore — useGridScrollRestoration saves scrollTop (in a module-scoped Map keyed by dagId, which survives the remount) on scroll, and restores it once, after the rows exist, in a useLayoutEffect (before
  paint, no flash). Covers both the plain-Grid and the shared Grid + Gantt scroll containers.
• Reset on leave — useResetGridScrollOnLeave (called once from the always-mounted BaseLayout) clears the store whenever the route is not a Dag detail path (/dags/<id>…). So scroll is kept only within a Dag; leaving to
  the Dags list / home / assets and returning starts at the top.

Files: useGridScrollRestoration.ts (+ test), Grid.tsx (wire-up), and one line in BaseLayout.tsx (reset-on-leave). Routing and the panel layout are untouched.

Alternative considered (not taken)

Make DetailsLayout a single shared parent route so the Grid mounts once and persists across Dag ↔ Run ↔ Task ↔ TaskInstance (the "correct" React Router structure), with per-page content flowing through the existing <Outlet/>.

Rejected as disproportionate: it is a large refactor with real regression risk.

Testing

New unit tests cover:

• Persist/restore: save-on-scroll, restore-once-rows-exist, deferred restore when rows arrive later, restore fires only once (does not clobber later user scrolling), no-op when there is no saved offset / a zero offset / a missing scroll element, and listener cleanup on unmount.
• Reset-on-leave: isDagDetailsPath for detail vs non-detail paths, clearGridScrollOffsets drops every entry, and the hook clears on a non-detail path but keeps offsets while inside a Dag (via MemoryRouter).

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)
  Co-authored-by: [Claude Opus 5] following the guidelines

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