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FA-8806 / Viewport layout / Open access

Sticky element container constraints: Sticky position ignores scroll displacement · case 01

Sticky position ignores scroll displacement.

Verified by executionVariant 1 · 7 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The scroll coordinate operation uses `normal` where the contract requires `normal - scroll`.

VERIFIED REPAIR

Implement the scroll coordinate operation as `normal - scroll`.

Unsuccessful approach: Adding scroll moves content in the wrong direction.

Case contract

Convert normal and container positions to viewport coordinates; pin at inset until the containing block bottom, accounting for element height; report whether position differs from natural flow.

Why this case matters

A deterministic model of sticky element container constraints; 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(normal, scroll, inset, height, container_end):
    natural = normal
    sticky = max(natural, inset)
    limit = container_end - scroll - height
    position = min(sticky, limit)
    return (position, position != natural)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('natural', solve(50, 10, 5, 20, 200), (40, False))
check('pinned', solve(50, 60, 5, 20, 200), (5, True))
check('container exits', solve(50, 180, 5, 20, 200), (0, True))
check('inset', solve(30, 40, 12, 10, 100), (12, True))
check('at natural', solve(50, 45, 5, 20, 200), (5, False))
check('parameterized sticky geometry', solve(5*N,6*N,N,2*N,20*N), (N,True))
for repetition in range(N):
    check('repeat at natural', solve(50, 45, 5, 20, 200), (5, False))
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 fixtureActualExpectedOutcome
natural[50, False][40, False]Failed
pinned[50, False][5, True]Failed
container exits[0, True][0, True]Passed
inset[30, False][12, True]Failed
at natural[50, False][5, False]Failed
parameterized sticky geometry[5, False][1, True]Failed
repeat at natural[50, False][5, False]Failed

SHA-256 / 9599a34d9b95da7d3fbe70c62cf8989f09af4e1e4b7d57ddc67f4b971d6411b9

2 / The unsuccessful fix

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(normal, scroll, inset, height, container_end):
    natural = normal + scroll
    sticky = max(natural, inset)
    limit = container_end - scroll - height
    position = min(sticky, limit)
    return (position, position != natural)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('natural', solve(50, 10, 5, 20, 200), (40, False))
check('pinned', solve(50, 60, 5, 20, 200), (5, True))
check('container exits', solve(50, 180, 5, 20, 200), (0, True))
check('inset', solve(30, 40, 12, 10, 100), (12, True))
check('at natural', solve(50, 45, 5, 20, 200), (5, False))
check('parameterized sticky geometry', solve(5*N,6*N,N,2*N,20*N), (N,True))
for repetition in range(N):
    check('repeat at natural', solve(50, 45, 5, 20, 200), (5, False))
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 fixtureActualExpectedOutcome
natural[60, False][40, False]Failed
pinned[110, False][5, True]Failed
container exits[0, True][0, True]Passed
inset[50, True][12, True]Failed
at natural[95, False][5, False]Failed
parameterized sticky geometry[11, False][1, True]Failed
repeat at natural[95, False][5, False]Failed

SHA-256 / 3f4b21aeea0e5c7ca82fad77be59c8313cdc044c08d4b2b2c2acfe42617a78c5

3 / The verified repair

Exit 0
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(normal, scroll, inset, height, container_end):
    natural = normal - scroll
    sticky = max(natural, inset)
    limit = container_end - scroll - height
    position = min(sticky, limit)
    return (position, position != natural)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('natural', solve(50, 10, 5, 20, 200), (40, False))
check('pinned', solve(50, 60, 5, 20, 200), (5, True))
check('container exits', solve(50, 180, 5, 20, 200), (0, True))
check('inset', solve(30, 40, 12, 10, 100), (12, True))
check('at natural', solve(50, 45, 5, 20, 200), (5, False))
check('parameterized sticky geometry', solve(5*N,6*N,N,2*N,20*N), (N,True))
for repetition in range(N):
    check('repeat at natural', solve(50, 45, 5, 20, 200), (5, False))
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 fixtureActualExpectedOutcome
natural[40, False][40, False]Passed
pinned[5, True][5, True]Passed
container exits[0, True][0, True]Passed
inset[12, True][12, True]Passed
at natural[5, False][5, False]Passed
parameterized sticky geometry[1, True][1, True]Passed
repeat at natural[5, False][5, False]Passed

SHA-256 / 7ac71cf6266177ce4adfbf00c16606201fd7822edec3c31c5cc9873a2742c8b5

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 2026-09-29T14:38:24.493557+00:00.

Case digest / c9e124a34565349510dee4fb4d08e94f3a2f2a0bc60ae7ed8ae59d8acd19ca47