Scroll-Driven Portal Traversal

What this builds

A scroll section where the viewer approaches a portal, passes through it, and arrives in a visually different world, with scroll as the only input. The crossing is reversible: scrolling back up returns through the portal without the scene state getting stuck.

• Three-act pacing: approach, crossing, arrival, each with its own scroll budget
• A committed crossing point so the transition never sits half-finished
• Scene handoff that is idempotent and survives scrubbing in both directions
• Correct behaviour on scroll restoration and deep links into the middle of the section
• A DOM-only fallback that does not require WebGL

When to use / when not to