A rail that does not drift

Steps

1. 1 Guest The home rail 0:01 ▣

   Plan 52 built it, plan 84 made it swipe-first, plan 89 filled it. Everything below is arithmetic and timing inside that.

2. 2 Guest Travel past the loop seam, twice 0:03 ▣

   Sixteen pages of travel through the wrap. Every rest lands flush — the grid is anchored to where the rail IS, not to index 0.

3. 3 Guest The moment it loops, nothing happens 0:12 ▣

   This is the defect you could SEE: scrollLeft was assigned while smooth scrolling was live, so Chrome animated the wrap across the whole clone buffer.

4. 4 Guest Tap page 3 0:13 ▣

   The owner's 'jumps to the next few events', made visible — and found broken here, not in the tests: scroll-snap-stop forbids a programmatic scroll passing a snap point, so every dot moved exactly one page.

5. 5 Guest Scroll a mouse wheel over it 0:15 ▣

   A vertical wheel over a horizontal container is ignored by every engine, so a desktop mouse user's only paths were drag and the arrows. At the ends the page keeps its own scroll — never swallowed.