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Browser-style history transitions: A pushed page is not selected as the current location · case 01

A pushed page is not selected as the current location.

Verified by executionVariant 1 · 9 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The push index operation uses `[url], index)` where the contract requires `[url], index+1)`.

THE FAILURE

The push index operation uses `[url], index)` where the contract requires `[url], index+1)`.

Unsuccessful approach: Resetting to the first entry displays the oldest page.

Case contract

Push truncates forward history and selects the new entry; replace retains stack length; back and forward clamp to existing entries; reload leaves history intact.

Why this case matters

A deterministic model of browser-style history transitions; 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(entries, index, action, url):
    entries = list(entries)
    if action == 'push': return (entries[:index+1] + [url], index)
    if action == 'replace': entries[index] = url
    elif action == 'back': index = max(0, index-1)
    elif action == 'forward': index = min(len(entries)-1, index+1)
    elif action == 'reload': return (entries, index)
    return (entries, index)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('branch', solve(['a','b','c'], 1, 'push', 'x'), (['a', 'b', 'x'], 2))
check('replace', solve(['a','b','c'], 1, 'replace', 'x'), (['a', 'x', 'c'], 1))
check('first back', solve(['a','b'], 0, 'back', None), (['a', 'b'], 0))
check('back', solve(['a','b'], 1, 'back', None), (['a', 'b'], 0))
check('last forward', solve(['a','b'], 1, 'forward', None), (['a', 'b'], 1))
check('forward', solve(['a','b'], 0, 'forward', None), (['a', 'b'], 1))
check('reload', solve(['a','b'], 1, 'reload', None), (['a', 'b'], 1))
check('parameterized push depth', solve(list(range(N+1)), N, 'push', N+1), (list(range(N+2)), N+1))
for repetition in range(N):
    check('repeat reload', solve(['a','b'], 1, 'reload', None), (['a', 'b'], 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 fixtureActualExpectedOutcome
branch[['a', 'b', 'x'], 1][['a', 'b', 'x'], 2]Failed
replace[['a', 'x', 'c'], 1][['a', 'x', 'c'], 1]Passed
first back[['a', 'b'], 0][['a', 'b'], 0]Passed
back[['a', 'b'], 0][['a', 'b'], 0]Passed
last forward[['a', 'b'], 1][['a', 'b'], 1]Passed
forward[['a', 'b'], 1][['a', 'b'], 1]Passed
reload[['a', 'b'], 1][['a', 'b'], 1]Passed
parameterized push depth[[0, 1, 2], 1][[0, 1, 2], 2]Failed
repeat reload[['a', 'b'], 1][['a', 'b'], 1]Passed

SHA-256 / 658b891a21572176516adc869aebd2b03812b2a4e2ea6405685ce87e4547d9a9

2 / The unsuccessful fix

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

N = 1
observations = []
def solve(entries, index, action, url):
    entries = list(entries)
    if action == 'push': return (entries[:index+1] + [url], 0)
    if action == 'replace': entries[index] = url
    elif action == 'back': index = max(0, index-1)
    elif action == 'forward': index = min(len(entries)-1, index+1)
    elif action == 'reload': return (entries, index)
    return (entries, index)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('branch', solve(['a','b','c'], 1, 'push', 'x'), (['a', 'b', 'x'], 2))
check('replace', solve(['a','b','c'], 1, 'replace', 'x'), (['a', 'x', 'c'], 1))
check('first back', solve(['a','b'], 0, 'back', None), (['a', 'b'], 0))
check('back', solve(['a','b'], 1, 'back', None), (['a', 'b'], 0))
check('last forward', solve(['a','b'], 1, 'forward', None), (['a', 'b'], 1))
check('forward', solve(['a','b'], 0, 'forward', None), (['a', 'b'], 1))
check('reload', solve(['a','b'], 1, 'reload', None), (['a', 'b'], 1))
check('parameterized push depth', solve(list(range(N+1)), N, 'push', N+1), (list(range(N+2)), N+1))
for repetition in range(N):
    check('repeat reload', solve(['a','b'], 1, 'reload', None), (['a', 'b'], 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 fixtureActualExpectedOutcome
branch[['a', 'b', 'x'], 0][['a', 'b', 'x'], 2]Failed
replace[['a', 'x', 'c'], 1][['a', 'x', 'c'], 1]Passed
first back[['a', 'b'], 0][['a', 'b'], 0]Passed
back[['a', 'b'], 0][['a', 'b'], 0]Passed
last forward[['a', 'b'], 1][['a', 'b'], 1]Passed
forward[['a', 'b'], 1][['a', 'b'], 1]Passed
reload[['a', 'b'], 1][['a', 'b'], 1]Passed
parameterized push depth[[0, 1, 2], 0][[0, 1, 2], 2]Failed
repeat reload[['a', 'b'], 1][['a', 'b'], 1]Passed

SHA-256 / f767678d7fc620e82fe62690a0b08b9618651df274ee6bdb4c84f3372602d1c4

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 9 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 2026-09-29T14:38:18.055582+00:00.

Case digest / 48b64bd29785c13615fcffa2028252b7c4cc1c46af8ea0d75cb4540a0364beca