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>
Two of the deferred scroll-jitter seams, on top of the integrated fix train
(T1.1 gate -> T2 writer -> T3 estimator @ 1fa4551a):
- columnWidthPx: measure the timeline row column once (useElementWidth) and
thread it into timelineRowReserveStyle so row-height reserves use the real
wrap width instead of the 64-char fallback. Pre-measure zero passes undefined
to preserve the estimator's own fallback rather than tripping its min-chars
floor. On jitter-corpus this shrinks estimate-error R from 75 -> 55 peak
(better reserves = smaller realized-vs-reserved delta) while the motion gate
stays GREEN 0.00/0.00 -- R moves only in the estimator lane, the gate only in
the writer lane, cross-lane invariant intact.
- Wren #4 anchor-contract guard: a companion perf test that reads the PRODUCTION
computed overflow-anchor on the real conversation scroller (no test override)
and asserts it resolves to none. The jitter gate forces the property for its
measurement; this proves the shipped stylesheet actually ships it, so a
regression that hands correction back to Chromium's native anchoring (masking
a WKWebView-only on-device break) fails loudly.
The onRealizedHeight seam is intentionally NOT wired: T4 ground truth
(RESEARCH/GUI_SCROLL_RECLAIM_T4.md) shows the upscroll path is append-only with
no unmount, so it needs no JS height cache; a per-event-id cache is only the
deferred, narrowly-scoped T4b for the head-refetch case. Adding an unused
callback now would be dead abstraction.
Validation:
- pnpm --dir desktop typecheck: clean
- biome check on both changed files: clean
- rowHeightEstimate.test.mjs 16/16, useAnchoredScroll.test.mjs 2/2
- perf: upscroll-jitter GREEN 0.00/0.00 (R 55), anchor-contract PASS
Co-authored-by: Tyler Longwell <tlongwell@block.xyz>
Signed-off-by: Tyler Longwell <tlongwell@block.xyz>
Dawn found the RED gate from 33e73511 was mathematically invariant to the
fix: its residual subtracted the REALIZED scrollTop delta, which includes
the fix-writer's compensating scrollBy, so the writer cancelled out and the
gate scored raw estimate error R (only T3 can move it), not motion
smoothness. Quinn and Eva re-derived and confirmed. Co-visible and any
row-vs-row differential share the disease: compensation is a uniform global
offset, so it cancels out of any quantity not referenced to the input.
Reframe (converged with Dawn/Quinn/Eva; ratified event 63f5e6e1):
- Metric is now per-notch MOTION CONSISTENCY: the reading row's viewport
motion (rowMove = after.top - before.top; NO scrollTop reference) scored
as deviation from the run median. This references the row's own motion
ACROSS notches (temporal self-reference), never a neighbour (spatial) or
scrollTop, so the writer's scrollBy survives into the metric and is SEEN.
- Actuation is a fixed synchronous step (scrollTop -= STEP) so the input
delta is constant by construction, bypassing Blink's wheel-scaling (a
wheel notch applies 218/220/222...; median-of-run would misread that
per-notch scaling as jitter). Dawn independently confirmed even the
actuated scrollTop delta is post-writer contaminated (48-75px spread
under sync) — rowMove is the only clean measurable.
- Anti-cheat floor: mean rowMove must be > STEP*0.75 so a frozen or
half-applying scroller (near-zero variance, would false-green) is caught.
- Old scrollTop-referenced residual kept as a printed NON-GATING diagnostic
(= estimate error R = T3's estimator acceptance number).
- Second non-gating diagnostic: one wheel-actuated pass of the same metric
(Tyler's real input is a wheel), on the record every run.
- RED-at-tip is a HARD gate with a VOID comment: median-of-run is only
valid while the corpus produces varying realization, and the tip run
being RED IS the proof of that dispersion.
RED at tip: peak-dev 41px / rms 24px (gate <=2 / <=0.6). GREEN target is
Dawn's real T2 writer: peak-dev 0.00 / rms 0.00, reproducible over 3 sync
runs. A synthetic per-notch-varying oracle drives 74/77 notches to exactly
STEP; the real writer re-pins in the same ResizeObserver cycle and closes
the synthetic-only outliers.
Co-authored-by: Tyler Longwell <tlongwell@block.xyz>
Signed-off-by: Tyler Longwell <tlongwell@block.xyz>
Tyler's report: scrolling UP a fully-loaded channel is subtly jumpy;
DOWN is smooth. Root cause (Eva H1 + L-C): rows above the opening
viewport have never painted, so under content-visibility:auto they sit
at estimateRowHeight()'s contain-intrinsic-size reserve (60px,
utilities.css:17). Scrolling up, each realizes at its true height; the
reserve-vs-true delta shifts the reading position. Shipped WKWebView has
no overflow-anchor, so it corrects nothing — every delta lands raw.
This adds a deterministic gate that is RED at tip 77bd0e70 and that the
converged owned-compensation fix (overflow-anchor:none + same-frame
scrollBy(realized-reserved)) turns green:
- e2eBridge: a dedicated `jitter-corpus` mock channel (400 structurally
heterogeneous rows — headings/lists/blockquotes/code fences/long prose
whose true height estimateRowHeight is known to miss), its own channel
so the uniform deep-history seed the load-older specs use is untouched.
- upscroll-jitter.perf.ts: forces overflow-anchor:none on the scroller
(mirrors shipped WKWebView; logs CSS.supports per engine), then wheels
up in notches tracking a row held INSIDE the viewport (SAFE_MARGIN band
from both edges, re-picked each step, scored only if the same id stayed
in-band before+after). Residual = row's visual delta-top minus the
scroll delta: 0 == smooth. Gate is peak <=2px, rms <=0.6px.
Validity: within a single cold run the already-realized rows read
residual 0.0 exactly while realizing rows drift -27..-75px — an artifact
would drift on both. Negative sign shows the estimator over-reserves.
At tip: peak 75px, rms 40px over 23 steps → RED. A correct fill of
scrollTop absorbs the drift into the scroll delta, driving residual->0.
Co-authored-by: Tyler Longwell <tlongwell@block.xyz>
Signed-off-by: Tyler Longwell <tlongwell@block.xyz>
