pl.Expr.get(pl.Expr.arg_min()) does not respect rolling window

[dalejung]

pl.Expr.get(pl.Expr.arg_min()) does not respect rolling window

[dalejung]

This also appears to be an issue for group_by_dynamic when defining different every and period.

dates = pl.date_range(
        pl.datetime(2023, 1, 1),
        pl.datetime(2024, 1, 1),
        eager=True,
    )
    df = pl.DataFrame({
        'date': dates,
    }).with_columns(
        num=pl.int_range(pl.len())
    )

    assert isinstance(df, pl.DataFrame)

    grouped = df.group_by_dynamic(
        'date',
        every='1mo',
        period='2mo',
    )

    report = grouped.agg(
        pl.col('num').first().alias('num_first'),
        pl.col('num').get(
            pl.col('num').arg_min(),
        ).alias('num_get'),
    )

In this example num_first and num_get should be the same but they are not. Removing period correctly shows them being the same.

[dalejung]

Possible that internally it is the same bug as:

#15091

I thought group_by_dyamic behaved better but now seeing that offset will return incorrect results like rolling, it feels similar.