What

Ninth PR in the flat inspector stack (see #2120 for the foundation and full stack list). Collapsed group headers now render in fixed, non-scrolling document flow above and below the open group — always fully visible, in natural order. Only the open group's own body content scrolls, in a dedicated scrollable region between the two fixed header stacks. Also fixes the flat inspector footer's missing background (was fully transparent).

Stack: #2120 → ... → #2127 → #2135 (this).

Why

An earlier position: sticky-based approach (collapsed headers stacking to the top/bottom edges as you scroll one shared container) was tried and refined live, but required sub-pixel-precision offset math and still hit a Chromium rounding quirk at the sticky-to-footer boundary. The user watched this live and asked for something architecturally simpler: headers that never move at all, with only the expanded section's content scrolling. This PR is that simpler design — collapsed here into one clean commit rather than shipping the sticky approach and then reverting it in a follow-up.

How

• PropertyPanelFlat.tsx splits the group list into beforeOpen / openGroup / afterOpen based on the open group's position. beforeOpen and afterOpen render as plain collapsed headers in normal flow; openGroup renders its header (also fixed) followed by a dedicated overflow-y-auto div holding just its body content.
• FlatGroup split into a header-only FlatGroupHeader component — no more children/body prop, no position: sticky, no offset math.
• PropertyPanelFlatFooter.tsx gained a bg-panel-bg background (it was fully transparent, a real pre-existing bug independent of any scrolling approach).
• Because nothing scrolls except the one region that's supposed to, this eliminates the whole class of sub-pixel/rounding bugs the sticky approach was fighting — there's no seam for content to bleed through.

Test plan

• New tests confirm the three-region DOM ordering (before-open headers → open group's header + scrollable body → after-open headers) and the "nothing open" fallback (all groups render as plain collapsed headers, no empty scrollable region).
• Verified live in the browser: measured getBoundingClientRect().top of every collapsed header and the footer before and after scrolling the open group's body — values were bit-for-bit identical, confirming headers genuinely never move.
• Full monorepo suite green (1587 tests, 0 failures); oxlint/oxfmt/fallow clean.
• Known follow-up, not fixed here: pinning several groups with long content plus several collapsed headers can exceed the panel's height with no scroll fallback (content silently clips). Resolved by the next PR in this stack, which removes group pinning entirely.
• Unit tests added/updated
• Manual testing performed
• Documentation updated (not applicable — internal Studio UI behind an off-by-default flag)