FA-8326 / Viewport layout / Open access

Virtualized row visibility: Overscan fails to include rows after the viewport · case 01

Overscan fails to include rows after the viewport.

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

ROOT CAUSE

The after overscan operation uses `visible[-1] + 1` where the contract requires `min(len(heights), visible[-1] + 1 + overscan)`.

THE FAILURE

The after overscan operation uses `visible[-1] + 1` where the contract requires `min(len(heights), visible[-1] + 1 + overscan)`.

Unsuccessful approach: Omitting the exclusive-end increment drops the final requested row.

Case contract

Rows overlap a half-open viewport using cumulative measured heights; overscan expands both sides and clamps to valid row indices.

Why this case matters

A deterministic model of virtualized row visibility; 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(heights, offset, viewport, overscan):
    positions = [0]
    for height in heights: positions.append(positions[-1] + height)
    visible = [i for i in range(len(heights)) if positions[i+1] > offset and positions[i] < offset + viewport]
    if not visible: return []
    lo = max(0, visible[0] - overscan)
    hi = visible[-1] + 1
    return list(range(lo, hi))
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('variable', solve([10,30,5], 15, 10, 0), [1])
check('top edge', solve([10,10,10], 10, 10, 0), [1])
check('partial top', solve([10,10,10], 5, 10, 0), [0, 1])
check('partial bottom', solve([10,10,10], 0, 15, 0), [0, 1])
check('overscan', solve([10]*6, 20, 10, 1), [1, 2, 3])
check('outside', solve([10,10], 30, 5, 1), [])
check('parameterized measured heights', solve([N]*6, 2*N, N, 1), [1,2,3])
for repetition in range(N):
    check('repeat outside', solve([10,10], 30, 5, 1), [])
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
variable	`[1]`	`[1]`	Passed
top edge	`[1]`	`[1]`	Passed
partial top	`[0, 1]`	`[0, 1]`	Passed
partial bottom	`[0, 1]`	`[0, 1]`	Passed
overscan	`[1, 2]`	`[1, 2, 3]`	Failed
outside	`[]`	`[]`	Passed
parameterized measured heights	`[1, 2]`	`[1, 2, 3]`	Failed
repeat outside	`[]`	`[]`	Passed

SHA-256 / 0b050248feece8cd35a26dbc5ab8f2a0a45e48e221dbc0637c9d41125a16a50b

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(heights, offset, viewport, overscan):
    positions = [0]
    for height in heights: positions.append(positions[-1] + height)
    visible = [i for i in range(len(heights)) if positions[i+1] > offset and positions[i] < offset + viewport]
    if not visible: return []
    lo = max(0, visible[0] - overscan)
    hi = min(len(heights), visible[-1] + overscan)
    return list(range(lo, hi))
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('variable', solve([10,30,5], 15, 10, 0), [1])
check('top edge', solve([10,10,10], 10, 10, 0), [1])
check('partial top', solve([10,10,10], 5, 10, 0), [0, 1])
check('partial bottom', solve([10,10,10], 0, 15, 0), [0, 1])
check('overscan', solve([10]*6, 20, 10, 1), [1, 2, 3])
check('outside', solve([10,10], 30, 5, 1), [])
check('parameterized measured heights', solve([N]*6, 2*N, N, 1), [1,2,3])
for repetition in range(N):
    check('repeat outside', solve([10,10], 30, 5, 1), [])
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
variable	`[]`	`[1]`	Failed
top edge	`[]`	`[1]`	Failed
partial top	`[0]`	`[0, 1]`	Failed
partial bottom	`[0]`	`[0, 1]`	Failed
overscan	`[1, 2]`	`[1, 2, 3]`	Failed
outside	`[]`	`[]`	Passed
parameterized measured heights	`[1, 2]`	`[1, 2, 3]`	Failed
repeat outside	`[]`	`[]`	Passed

SHA-256 / f581061538fcf82baa08d369d9e46359b05e6ac99fff1ba603de9a1597ee042e

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 2026-09-29T14:38:20.368588+00:00.

Case digest / 72f0c297c07a51757f3ba6f7cff8f87b0b6d0e9c9ff28d1f1498aa277d7be4a9