[pushpsood]

Summary

/messenger

• Each screen's animation plays once, for the same 5 s on every screen, the first time it arrives going forward. Scroll speed never speeds it up or cuts it short.
• The stage walks one screen at a time and never skips. Screens already seen, or reached by scrolling up, switch straight to their finished state and stay 0.7–2 s depending on scroll speed.
• The page cannot scroll past the next section while a screen is on stage.
• The footer appears only after screen 8 has had its time; the dark closer between screen 8 and the footer is removed from this page.
• Screen 7 loads a second wave of real messenger features (voice/video calls, reactions, disappearing messages, safety code, read receipts, ...).
• Screen 6: Mr OneHook's reply follows the payload with no gap; the sparkle ring no longer replays on revisits (new settled prop on AlienScanner / AiSparkle).
• Screen 4 → 5: the phone leaves first, then the caption window grows from where it vanished; no empty wiring frame.
• Less per-frame work: no scroll-scrub follower loop, finished screens are static, ambient loops on revisited screens start stopped.

/science

• Floating bottom bar becomes a horizontal chapter "drum" (drag or scroll to jump chapters), following the Get-the-app bar's show/hide rules.
• New ScienceOverview section; Finale gains compact and showTitle; diagram, lab and hero refinements.

Testing

• npm run lint, npm test (521 passed, including the new messenger-story-timeline tests for play-once, no-skip, speed-relative dwell and the scroll cap), npm run build.
• Messenger behaviour verified in headless Chrome at desktop size: per-screen timings, the scroll cap (wheel, End key, scrollbar jump), footer gating, and frame-by-frame films of every switch from 04 to 08.

Not verified

• Touch / momentum scrolling on real iOS and Android devices (scroll cap and dwell timing).
• The Science-page changes were made outside this session; they pass type-check, tests and build, but I did not review their behaviour in a browser.