Back/forward scroll restore: drop the pre-swap popstate listener that corrupted saved offsets

[joestump]

Part of #190.

Fixes the back/forward reading-position corruption introduced with the #197 scroll-restore mechanism (shell.js).

Root cause

restoreScroll was registered on popstate in addition to htmx:historyRestore. shell.js loads before htmx.min.js (both defer, template order in partials.html), so its popstate listener fires before htmx's own — while the departing page is still mounted in #main-content but location has already flipped to the destination. It therefore scrolled the departing page to the destination's saved offset (clamped to the departing page's max). htmx's popstate handler then fired htmx:beforeHistorySave for the departing page, and saveScroll persisted that corrupted value under the departing page's key.

Net effect, deterministically: scroll /c/N to 400, boosted-nav to /media, then Back → /media's key corrupts 0→24; Forward → /media lands at 24 and /c/N's key clobbers 400→24; Back again → reading position gone.

Fix

Drop the popstate binding; htmx:historyRestore is the only restore trigger. The popstate listener covered nothing: the vendored htmx 2.0.4 fires htmx:historyRestore after the swap on both history-cache paths — cache hit and server-refetch miss ({cacheMiss: true}) — and on popstates htmx ignores (non-htmx state entries) no swap happens, so touching the still-mounted page's scroller is exactly the corruption above. The block comment now documents why popstate must never restore.

TestShellScrollRestoreBindings pins the wiring statically: restore bound to htmx:historyRestore only, exactly one popstate listener (the idempotent tab-state sync), and saveScroll keyed by detail.path.

Verification (headless Chrome CDP, fixture archive, before → after)

5-step repro — scroll /c/2 to 400, boosted nav to /media, then traverse history:

Step	main (ccda63f)	this branch
Back → /c/2	400 ✅ but /media key 0→24 ❌	400 ✅, /media key stays 0 ✅
Forward → /media	lands 24 ❌, /c/2 key 400→24 ❌	lands 0 ✅, /c/2 key stays 400 ✅
Back → /c/2	lands 24 — position lost ❌	400 ✅

Cache-miss coverage: with htmx-history-cache forcibly cleared on /media, Back refetched /c/2 from the server and htmx:historyRestore fired with cacheMiss: true — scroller restored to 400, so the dropped listener loses no coverage on the no-snapshot path.

Hygiene: rm -rf .tools && make css leaves the committed app.css byte-identical (drift guard green); full go test ./... passes.

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🤖 Posted on behalf of @joestump by Claude.