FA-8281 / Viewport layout / Open access

Nearest-edge scroll reveal: An oversized item reveals its end instead of its leading edge · case 01

An oversized item reveals its end instead of its leading edge.

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

ROOT CAUSE

The oversized operation uses `if end - start > viewport: wanted = end - viewport` where the contract requires `if end - start > viewport: wanted = start`.

THE FAILURE

The oversized operation uses `if end - start > viewport: wanted = end - viewport` where the contract requires `if end - start > viewport: wanted = start`.

Unsuccessful approach: Leaving offset unchanged may reveal none of the target.

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 = end - viewport
    elif start < offset: wanted = start
    elif end > offset + viewport: wanted = end - viewport
    else: wanted = offset
    return max(0, min(wanted, max(0, extent - viewport)))
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	`20`	`10`	Failed
above	`10`	`10`	Passed
below	`25`	`25`	Passed
visible	`10`	`10`	Passed
end clamp	`80`	`80`	Passed
negative clamp	`0`	`0`	Passed
parameterized reveal distance	`5`	`5`	Passed
repeat negative clamp	`0`	`0`	Passed

SHA-256 / 875456309261128500a5e72924ab762f2ae94d46f6c5c11c1f9f99cd5552775d

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 = offset
    elif start < offset: wanted = start
    elif end > offset + viewport: wanted = end - viewport
    else: wanted = offset
    return max(0, min(wanted, max(0, extent - viewport)))
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	`0`	`10`	Failed
above	`10`	`10`	Passed
below	`25`	`25`	Passed
visible	`10`	`10`	Passed
end clamp	`80`	`80`	Passed
negative clamp	`0`	`0`	Passed
parameterized reveal distance	`5`	`5`	Passed
repeat negative clamp	`0`	`0`	Passed

SHA-256 / 20246f88dc4f3e48616c1ef1760f228b29fd8bbc35c0c21e57d4dd7e6d16f9fe

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.

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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 / 3eebcf694ba708c234cd95650a45c1ddc19f775f042b38e58115772e02a1c92e