FA-7961 / Selection controls / Open access

Anchored range selection: An ordinary click unexpectedly extends from the old anchor · case 01

An ordinary click unexpectedly extends from the old anchor.

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

ROOT CAUSE

The plain click operation uses `anchor not in items` where the contract requires `not extend or anchor not in items`.

THE FAILURE

The plain click operation uses `anchor not in items` where the contract requires `not extend or anchor not in items`.

Unsuccessful approach: Always treating clicks as plain prevents shift ranges.

Case contract

Ranges include both endpoints in document order; plain clicks replace; additive selection unions; missing targets preserve selection.

Why this case matters

A deterministic model of anchored range selection; 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(items, selected, anchor, target, extend, additive):
    if target not in items: return sorted(selected)
    if anchor not in items: span = {target}
    else:
        a, b = sorted((items.index(anchor), items.index(target)))
        span = set(items[a:b+1])
    return sorted(set(selected) | span if additive else span)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('removed', solve(['a','b'], ['a'], 'a', 'x', False, False), ['a'])
check('plain', solve(['a','b','c'], ['a'], 'a', 'c', False, False), ['c'])
check('forward', solve(['a','b','c'], [], 'a', 'c', True, False), ['a', 'b', 'c'])
check('backward', solve(['a','b','c'], [], 'c', 'a', True, False), ['a', 'b', 'c'])
check('union', solve(['a','b','c'], ['a'], 'b', 'c', True, True), ['a', 'b', 'c'])
check('replace', solve(['a','b','c'], ['a'], 'b', 'c', True, False), ['b', 'c'])
check('parameterized inclusive range', solve(list(range(N+2)), [], 0, N, True, False), list(range(N+1)))
for repetition in range(N):
    check('repeat replace', solve(['a','b','c'], ['a'], 'b', 'c', True, False), ['b', 'c'])
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
removed	`['a']`	`['a']`	Passed
plain	`['a', 'b', 'c']`	`['c']`	Failed
forward	`['a', 'b', 'c']`	`['a', 'b', 'c']`	Passed
backward	`['a', 'b', 'c']`	`['a', 'b', 'c']`	Passed
union	`['a', 'b', 'c']`	`['a', 'b', 'c']`	Passed
replace	`['b', 'c']`	`['b', 'c']`	Passed
parameterized inclusive range	`[0, 1]`	`[0, 1]`	Passed
repeat replace	`['b', 'c']`	`['b', 'c']`	Passed

SHA-256 / 3ce64b7e9c92c8a8aed6c50dfe2b5336f7f33b7fe7c4dd966b142af526eb184b

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(items, selected, anchor, target, extend, additive):
    if target not in items: return sorted(selected)
    if True: span = {target}
    else:
        a, b = sorted((items.index(anchor), items.index(target)))
        span = set(items[a:b+1])
    return sorted(set(selected) | span if additive else span)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('removed', solve(['a','b'], ['a'], 'a', 'x', False, False), ['a'])
check('plain', solve(['a','b','c'], ['a'], 'a', 'c', False, False), ['c'])
check('forward', solve(['a','b','c'], [], 'a', 'c', True, False), ['a', 'b', 'c'])
check('backward', solve(['a','b','c'], [], 'c', 'a', True, False), ['a', 'b', 'c'])
check('union', solve(['a','b','c'], ['a'], 'b', 'c', True, True), ['a', 'b', 'c'])
check('replace', solve(['a','b','c'], ['a'], 'b', 'c', True, False), ['b', 'c'])
check('parameterized inclusive range', solve(list(range(N+2)), [], 0, N, True, False), list(range(N+1)))
for repetition in range(N):
    check('repeat replace', solve(['a','b','c'], ['a'], 'b', 'c', True, False), ['b', 'c'])
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
removed	`['a']`	`['a']`	Passed
plain	`['c']`	`['c']`	Passed
forward	`['c']`	`['a', 'b', 'c']`	Failed
backward	`['a']`	`['a', 'b', 'c']`	Failed
union	`['a', 'c']`	`['a', 'b', 'c']`	Failed
replace	`['c']`	`['b', 'c']`	Failed
parameterized inclusive range	`[1]`	`[0, 1]`	Failed
repeat replace	`['c']`	`['b', 'c']`	Failed

SHA-256 / f9850a90020e1cd73e37ff89298224a2d5087b07eca8a0f04d367cdbaff58987

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 2026-09-29T14:38:17.048216+00:00.

Case digest / ed20c8f7602f9a861250a4755ecee6a3f9a30412611976fb20d809468327a92c