FA-8301 / Viewport layout / Open access

Nearest-edge scroll reveal: Reveal requests scroll beyond the usable content extent · case 01

Reveal requests scroll beyond the usable content extent.

Verified by executionVariant 1 · 8 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The bounds operation uses `wanted` where the contract requires `max(0, min(wanted, max(0, extent - viewport)))`.

THE FAILURE

The bounds operation uses `wanted` where the contract requires `max(0, min(wanted, max(0, extent - viewport)))`.

Unsuccessful approach: Clamping to full extent ignores the viewport size.

Case contract

Keep already-visible items stationary; reveal nearest edge; align oversized items at the start; clamp offset to the scrollable extent.

Why this case matters

A deterministic model of nearest-edge scroll reveal; this isolates one interface invariant without requiring a browser.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(start, end, offset, viewport, extent):
    if end - start > viewport: wanted = start
    elif start < offset: wanted = start
    elif end > offset + viewport: wanted = end - viewport
    else: wanted = offset
    return wanted
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('oversized', solve(10, 40, 0, 20, 100), 10)
check('above', solve(10, 15, 30, 20, 100), 10)
check('below', solve(40, 45, 0, 20, 100), 25)
check('visible', solve(12, 15, 10, 20, 100), 10)
check('end clamp', solve(95, 105, 0, 20, 100), 80)
check('negative clamp', solve(-5, 0, 10, 20, 100), 0)
check('parameterized reveal distance', solve(10*N, 10*N+5, 0, 10, 100*N), 10*N-5)
for repetition in range(N):
    check('repeat negative clamp', solve(-5, 0, 10, 20, 100), 0)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
oversized	`10`	`10`	Passed
above	`10`	`10`	Passed
below	`25`	`25`	Passed
visible	`10`	`10`	Passed
end clamp	`85`	`80`	Failed
negative clamp	`-5`	`0`	Failed
parameterized reveal distance	`5`	`5`	Passed
repeat negative clamp	`-5`	`0`	Failed

SHA-256 / 6088879cd0836cb26c3399cba10b9e8db72d6a903c225ebff9516ede120afae3

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(start, end, offset, viewport, extent):
    if end - start > viewport: wanted = start
    elif start < offset: wanted = start
    elif end > offset + viewport: wanted = end - viewport
    else: wanted = offset
    return max(0, min(wanted, extent))
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('oversized', solve(10, 40, 0, 20, 100), 10)
check('above', solve(10, 15, 30, 20, 100), 10)
check('below', solve(40, 45, 0, 20, 100), 25)
check('visible', solve(12, 15, 10, 20, 100), 10)
check('end clamp', solve(95, 105, 0, 20, 100), 80)
check('negative clamp', solve(-5, 0, 10, 20, 100), 0)
check('parameterized reveal distance', solve(10*N, 10*N+5, 0, 10, 100*N), 10*N-5)
for repetition in range(N):
    check('repeat negative clamp', solve(-5, 0, 10, 20, 100), 0)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
oversized	`10`	`10`	Passed
above	`10`	`10`	Passed
below	`25`	`25`	Passed
visible	`10`	`10`	Passed
end clamp	`85`	`80`	Failed
negative clamp	`0`	`0`	Passed
parameterized reveal distance	`5`	`5`	Passed
repeat negative clamp	`0`	`0`	Passed

SHA-256 / fdfeed34fd54c844bab87dff32e61b83d6ceb48f38c52bd607022a61e8d56537

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 8 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

Member access is invitation-based. Sign in with your invited account to inspect the repair.

Verification & scope

This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:38:20.163896+00:00.

Case digest / 4c8f616389ba5c8d37fff0bc944916c213f93ab38950708b95ef7e7c2167c196