FA-8316 / Viewport layout / Open access

Virtualized row visibility: A row starting at the viewport bottom is rendered as visible · case 01

A row starting at the viewport bottom is rendered as visible.

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

ROOT CAUSE

The top boundary operation uses `positions[i] <= offset + viewport` where the contract requires `positions[i] < offset + viewport`.

THE FAILURE

The top boundary operation uses `positions[i] <= offset + viewport` where the contract requires `positions[i] < offset + viewport`.

Unsuccessful approach: Requiring the full row end excludes partially visible bottom rows.

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 = min(len(heights), visible[-1] + 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]`	`[1]`	Passed
top edge	`[1, 2]`	`[1]`	Failed
partial top	`[0, 1]`	`[0, 1]`	Passed
partial bottom	`[0, 1]`	`[0, 1]`	Passed
overscan	`[1, 2, 3, 4]`	`[1, 2, 3]`	Failed
outside	`[]`	`[]`	Passed
parameterized measured heights	`[1, 2, 3, 4]`	`[1, 2, 3]`	Failed
repeat outside	`[]`	`[]`	Passed

SHA-256 / 2a2a5644eab3bfaf08b47ddd26fe271193b362b8a51c31dd5cd74014452edb4a

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+1] <= offset + viewport]
    if not visible: return []
    lo = max(0, visible[0] - overscan)
    hi = min(len(heights), visible[-1] + 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]`	`[1]`	Passed
partial top	`[0]`	`[0, 1]`	Failed
partial bottom	`[0]`	`[0, 1]`	Failed
overscan	`[1, 2, 3]`	`[1, 2, 3]`	Passed
outside	`[]`	`[]`	Passed
parameterized measured heights	`[1, 2, 3]`	`[1, 2, 3]`	Passed
repeat outside	`[]`	`[]`	Passed

SHA-256 / e1c6c36bc25a58640d23f9a15d891c1d76628a86122cb61f84e0c0f93e12a422

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

Case digest / 2087c1a563912f8912c835c83b23a40c8b4cc7c80a172cfbe1d1896a95d9ab6e