What

Eleventh PR in the flat inspector stack (see #2120 for the foundation and full stack list). Adds a fast (~120ms) entrance animation when accordion group headers expand/collapse, matching this codebase's existing plain-CSS @keyframes-based animation convention (no new library added).

Stack: #2120 → ... → #2135 → #2136 (this).

Why

Requested directly — toggling groups was instant with no transition. Since collapsed/open headers are two different JSX branches (React mounts a fresh node on every toggle rather than morphing one persistent element), an entrance animation on whichever branch just mounted was the correct, minimal approach rather than restructuring the DOM for a true height-morph.

How

• New @keyframes hf-flat-group-in + .hf-flat-group-enter class in studio.css, mirroring the existing hf-toast-in pattern (opacity + small translateY, prefers-reduced-motion override).
• PropertyPanelFlat.tsx tracks justToggledIds: string[] — populated with both the clicked group's id and whichever group was implicitly closed by the click (deduped when they're the same), cleared ~200ms after each toggle — so only the two groups actually transitioning ever animate, not unrelated siblings.
• Caught during implementation, not shipped as a defect: the first pass applied the animation class unconditionally to whatever branch mounted, but a click jumping between non-adjacent groups (e.g. Motion→Text, skipping Style/Layout) caused those skipped-over siblings to incorrectly replay the animation too — a Chromium quirk where array-position shifts restart CSS animations even without a prop change on the affected element. Fixed by explicitly gating the class on justToggledIds rather than relying on incidental remounts.
• A follow-up review caught a second gap: the initial fix only tracked the clicked group, so a group closing implicitly (displaced by a different group opening) never got its own collapse-entrance animation. Fixed by capturing the previously-open group's id alongside the clicked one.

Test plan

• Component-level tests confirm the animation class renders correctly on both collapsed and open states.
• Integration-level regression test (added after the second review round) drives real clicks through a non-adjacent double-jump (Text→Motion→Text) and asserts untouched siblings never get the class while both the closing and opening groups do — verified via A/B revert that this test actually fails against the earlier single-id implementation.
• Verified live in the browser: clicked between non-adjacent groups and confirmed via direct DOM inspection that exactly the closing and opening group get the animation class, with all untouched siblings at false.
• Confirmed the animation is purely cosmetic — freshly-opened content is fully interactive immediately, nothing gates on the animation completing.
• Full monorepo suite green (1581 tests, 0 failures); oxlint/oxfmt/fallow clean.
• Unit tests added/updated
• Manual testing performed
• Documentation updated (not applicable — internal Studio UI behind an off-by-default flag)