collapsing-parallax-toolbar

Build a collapsing header from five siblings in one box — four sharing a single scroll state, one driven by a boolean instead — with artwork moved by a graphics layer at half the scroll rate, a title interpolated along a two-segment curve into the pinned bar, and a derived flip point that swaps the floating back button for a real top bar. Use when a parallax header jitters or re-measures while scrolling, when the…