FA-8306 / Viewport layout / Open access

Virtualized row visibility: Variable row heights are not accumulated into layout positions · case 01

Variable row heights are not accumulated into layout positions.

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

ROOT CAUSE

The cumulative heights operation uses `height` where the contract requires `positions[-1] + height`.

THE FAILURE

The cumulative heights operation uses `height` where the contract requires `positions[-1] + height`.

Unsuccessful approach: Assuming unit rows ignores measured row sizes.

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(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]`	Failed
partial top	`[0, 1, 2]`	`[0, 1]`	Failed
partial bottom	`[0, 1, 2]`	`[0, 1]`	Failed
overscan	`[]`	`[1, 2, 3]`	Failed
outside	`[]`	`[]`	Passed
parameterized measured heights	`[]`	`[1, 2, 3]`	Failed
repeat outside	`[]`	`[]`	Passed

SHA-256 / 973d51d82d6d439f1231bec28f03ccb1a7076fbfafc640d1e0e184b6a17ad461

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] + 1)
    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]`	Failed
top edge	`[]`	`[1]`	Failed
partial top	`[]`	`[0, 1]`	Failed
partial bottom	`[0, 1, 2]`	`[0, 1]`	Failed
overscan	`[]`	`[1, 2, 3]`	Failed
outside	`[]`	`[]`	Passed
parameterized measured heights	`[1, 2, 3]`	`[1, 2, 3]`	Passed
repeat outside	`[]`	`[]`	Passed

SHA-256 / 67829b358468149767bec5b4f60f50877d8027442ee435ebb7e73612544e723d

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 / c3c4fb3f836269c16ba9a1b8b1c336be080553a692ead1e1881168d53ce2e276