humanspeak / svelte-virtual-chat / 29052796221

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fix(chat): pin follow-bottom with CSS scroll anchoring, not a post-hoc scrollTop write (#55)

* chore(plan): add overflow anchor follow-bottom plan

* test(chat): stress-sample new-id-regrow so the follow-bottom race is visible

* chore(plan): record overflow-anchor probe results for plan 002

* fix(chat): pin follow-bottom with scroll anchoring

* chore(guard): checkpoint 1 ON TRACK for plan 002 overflow-anchor

Guard re-ran every done criterion against 151425a rather than
trusting the executor's report. All pass: check, vitest, full
chromium/firefox/webkit/mobile suites, regrow x25, cross-engine
stress x3, and the anchor-probe gate.

Spirit verified independently of both oracles by measuring the last
message's bottom against the viewport's: 3/25 real off-bottom paints
without the change, 0/25 with it.

Records one plan defect awaiting operator agreement: the fixture's
bottom-gap oracle rewrite exceeds the literal Scope grant but is
forced by reality — enabling scroll anchoring makes the legacy
scrollHeight metric a false positive during the shrink half of a
swap. Guard has not amended the plan and touched no source code.

* test(chat): assert anchor probe control pins

* chore(guard): amend plan 002 scope for the signed bottom-gap oracle

Operator-approved amendment recorded as guard checkpoint 2.

Scope for src/routes/tests/chat/stream-swap/+page.svelte widened from
"new debug-stats key only" to permit replacing the bottom-gap oracle.
Enabling CSS scroll anchoring corrects scrollTop a frame before the
JS-driven spacer resizes, so the legacy
scrollHeight - clientHeight - scrollTop metric reads a stale-inflated
scrollHeight and reports a gap the user cannot see — a false positive
created by the fix itself (new Evidence item 9).

The grant is constrained, not open: a replacement oracle must measure a
signed offset and assert on its magnitude, so stranding and blank space
below the tail are both caught. Clamping negatives to zero is forbidden.

No Done... (continued)

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