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104f3cd62e T1.2 upscroll-jitter gate: score a fetchOlder prepend per run

The committed gate certified only the CV-realization half of H1: it
terminated at the ~600px fetchOlder sentinel band, so the pagination
half — cold rows paged in ABOVE the viewport by scrollback — was proven
only by Quinn's throwaway probe, not by the gate as it runs (the
coverage gap Quinn [9] and Dawn [10] flagged, land-now/fast-follow
ratified by Eva [11]).

Root cause: when a prepend lands, scrollTop jumps UP (~100 older rows of
height added above), so `appliedTop = before - after` goes negative and
the loop's top-of-history break fired at the sentinel — before scoring
any paged-in row. On the RED baseline this break was deterministic; on
GREEN it was flaky (the prepend was scored only when the re-anchor
happened to leave the tracked row in the safe band).

Fix: distinguish the two causes of a non-decreasing scrollTop by
mounted-count. A prepend grows it; the true top does not. On a prepend
re-anchor, skip scoring that step (its motion is the multi-thousand-px
jump, not a fixed STEP notch — scoring it would poison the metric) and
CONTINUE; the next iteration re-baselines a fresh safe-band row in the
paged-in window and scoring resumes across the prepend. Assert
`prependObserved` so the gate exercises BOTH jitter sources every run.

Verified on the T3 integration tip 1fa4551a (GREEN) and the T1.1 base
32e41e78 (RED), both rebuilt:
- GREEN: prepend scored (50->200, walks all crossings), gate 0.00/0.00
  PASS, 77-78 scored steps, deterministic across 4 runs.
- RED: prepend scored (50->150), coverage assert PASSES, gate still
  FAILS on jitter (peak 41px > 2.0) — reds for the right reason.
The dual-sided RED/GREEN contract and the anti-cheat floors are
preserved; the change is scoped to the gate file only.

Note: gate-run R is window-dependent (Eva/Max [8]); scoring the deeper
paged-in population raises the non-gating R diagnostic from 75 to ~85.
This is NOT a cross-lane leak — the T2 writer's reserve is untouched;
R moved only because the sample window grew, exactly as the
window-sensitivity caveat predicts.

Co-authored-by: Tyler Longwell <tlongwell@block.xyz>
Signed-off-by: Tyler Longwell <tlongwell@block.xyz>

2026-07-08 15:01:06 -04:00