Fixture P07: the original component below. Steps are a procedure, not completed test results.
Reload: toggle is collapsed, panel inert. Tab/Enter opens; after500ms inspect height, scrollHeight, visibility, inert, focus and supported interpolate-size.
Tab into the panel; activate Add a packing note twice. Check two original notes and increased open height, without claiming the added-content size change animated.
Focus Close notes and activate: focus returns to toggle, expanded false, inert true immediately and height0 after500ms. Reopen, focus inner link, press Escape and check the same return path.
Rapidly toggle open-close-open; settle500ms and inspect a fully open panel. Select Use instant changes and repeat open/close; this forces the designed instant path, not an unsupported-browser result.
With panel closed, Tab through surrounding controls; inner link/buttons should not receive focus. Unsupported interpolate-size browser, real OS reduced-motion change, screen-reader output and physical touch remain untested.