feat(studio): sticky accordion group headers (stack top and bottom)

[vanceingalls]

What

Ninth PR in the flat inspector stack (see #2120 for the foundation and full stack list). Group headers in the flat accordion — collapsed groups, the currently-open group's own header, and pinned-group headers — now stick to the top/bottom edge of the scroll container while scrolling, macOS Finder-style: headers above the current scroll position stack at the top, headers below stack at the bottom.

Stack: #2120 → ... → #2127 → #2128 (this).

Why

Requested after live manual testing — scrolling a long open group's content (Grade, with 14+ rows) scrolled its own header out of view along with everything else, losing the ability to see which group you're in or collapse/unpin it without scrolling back up.

How

Pure CSS, no new component logic: position: sticky with both top: 0 and bottom: 0 set on each header naturally sticks to whichever edge is being approached (top scrolling down, bottom scrolling up), and multiple sibling headers with the same styling stack against the edge in DOM order automatically — the same mechanism behind Finder/iOS Settings section headers.

• Collapsed FlatGroup buttons, the open FlatGroup's header div, and PinnedGroupRow's header div all gained sticky top-0 bottom-0 z-10 bg-panel-bg.
• The two nested headers (open-group and pinned-group, both sitting inside a padded outer wrapper) needed a -mx-4 px-4 bleed so their opaque background spans the full row width instead of stopping at the parent's padding edge — the collapsed-button case didn't need this since it's already a direct, full-width child of the scroll container.
• Confirmed no intermediate ancestor between these headers and the outer scroll container has a competing overflow property that would break the sticky positioning.

Test plan

• This is a pure-styling change with no new interactive behavior, so tests are simple className assertions confirming the sticky/background/z-index classes landed on the correct specific elements (the header div, not the whole group), plus confirming every pre-existing interactive test (pin/collapse/unpin clicks) is unchanged.
• Important: neither the implementer nor I have browser access — the actual visual stacking/scroll behavior has NOT been visually verified and needs a live check in the running Studio dev server before considering this fully done. The CSS/DOM-nesting reasoning was independently re-verified in review as the best available substitute for a screenshot.
• Full monorepo suite green (1582 tests, 0 failures); oxlint/oxfmt/fallow clean.
• Unit tests added/updated
• Manual testing performed — pending live visual verification, flagged above
• Documentation updated (not applicable — internal Studio UI behind an off-by-default flag)

[vanceingalls]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(studio): sticky accordion group headers (stack top and bottom) #2128 👈 (View in Graphite)
• fix(studio): Style/Grade/Text parity gaps found in legacy-retirement audit #2127 : 2 other dependent PRs (#2129 , #2134 )
• fix(studio): retire legacy Style and Grade sections from flat inspector #2126
• feat(studio): flat inspector — persisted pinning + multi-field Text #2125
• feat(studio): flat inspector — Grade (color grading) group #2124
• feat(studio): flat inspector — Media group #2123
• feat(studio): flat inspector — Layout + Motion groups #2122
• feat(studio): flat inspector — Style group #2121
• feat(studio): flat inspector foundation + Text group #2120
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[miga-heygen]

Reviewed as part of the 15-PR edit-panel redesign stack. Full stack review posted on #2120. No blockers on this PR. — Miga

[miguel-heygen]

Final current-head pass: 9232cc8. Reviewed the stack boundaries and current diff against the flat-inspector contracts; required checks have no failures/pending checks and no unresolved review threads remain. No new drift found; residual notes are non-blocking.

[miguel-heygen]

Superseding earlier approval: the latest max-review pass found confirmed correctness issues in this flat-inspector stack (identity/selector-index state, hide-all write races, timing inference, slider pointer/keyboard/reset semantics, and duplicate React keys). Hold merge and require fixes plus re-review on the current stack head.