BUG: df.iloc[(slice,)] changed behavior in pandas 2.1.0rc0

[flying-sheep]

Pandas version checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of pandas.
• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd, numpy as np
df = pd.DataFrame(np.random.rand(10, 8))
print(df.iloc[(slice(None, None, 2),)].shape)

Issue Description

Since 2.1.0rc0, the code above slices the second axis instead of the first, and prints:

(10, 4)

Expected Behavior

I expect the code to slice the first axis and print:

(5, 8)

Installed Versions

INSTALLED VERSIONS

commit : 6e194cf
python : 3.10.12.final.0
python-bits : 64
OS : Linux
OS-release : 6.4.10-zen2-1-zen
Version : #1 ZEN SMP PREEMPT_DYNAMIC Sun, 13 Aug 2023 01:33:59 +0000
machine : x86_64
processor :
byteorder : little
LC_ALL : None
LANG : de_DE.UTF-8
LOCALE : de_DE.UTF-8

pandas : 2.1.0rc0
numpy : 1.24.4
pytz : 2023.3
dateutil : 2.8.2
setuptools : 68.1.0
pip : 23.2.1
Cython : None
pytest : 7.4.0
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.2
IPython : None
pandas_datareader : None
bs4 : None
bottleneck : None
brotli : None
dataframe-api-compat: None
fastparquet : None
fsspec : 2023.6.0
gcsfs : None
matplotlib : 3.7.2
numba : 0.57.1
numexpr : None
odfpy : None
openpyxl : 3.1.2
pandas_gbq : None
pyarrow : None
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : 1.11.2
snappy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
zstandard : None
tzdata : 2023.3
qtpy : None
pyqt5 : None

[rhshadrach]

git bisect shows

4f4184d90135c9e890d5124df547dcfff51bacec is the first bad commit
commit 4f4184d90135c9e890d5124df547dcfff51bacec
Author: Patrick Hoefler <[email protected]>
Date:   Thu May 4 19:43:38 2023 +0200

    Improve performance when selecting rows and columns (#53014)

cc @phofl

[phofl]

I’ll take a look, can you link the pr number as well in the future?

[rhshadrach]

Updated.

[phofl]

I've got to say I am very confused that we don't have tests for this ourselves