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FA-8826 / Viewport layout / Open access

Sticky element container constraints: Sticky elements remain pinned after their container ends · case 01

Sticky elements remain pinned after their container ends.

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

ROOT CAUSE

The container clamp operation uses `sticky` where the contract requires `min(sticky, limit)`.

VERIFIED REPAIR

Implement the container clamp operation as `min(sticky, limit)`.

Unsuccessful approach: Taking max pushes them to the container bottom prematurely.

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 - scroll
    sticky = max(natural, inset)
    limit = container_end - scroll - height
    position = sticky
    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[5, True][0, True]Failed
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 / 5e751b391f1506f1dfb3054b021c3c1b39909bb7706b4b3c299fd87467e0d0ca

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 = max(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[170, True][40, False]Failed
pinned[120, True][5, True]Failed
container exits[5, True][0, True]Failed
inset[50, True][12, True]Failed
at natural[135, True][5, False]Failed
parameterized sticky geometry[12, True][1, True]Failed
repeat at natural[135, True][5, False]Failed

SHA-256 / 02c65b95002fdfe905abf1331f8b4b2c330b5d450906c57e19a867cf88582d0c

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 2026-09-29T14:38:24.771005+00:00.

Case digest / 414522c313a10a483448258b76502b91c5780f11b29797005fdb3e6bdd39f1e9