isel with drop=True does not drop coordinates if using scalar DataArray as indexer

[headtr1ck]

What happened?

When using DataArray/Dataset.isel with drop=True with a scalar DataArray as indexer (see example) resulting scalar coordinates do not get dropped.
When using an integer the behavior is as expected.

What did you expect to happen?

I expect that using a scalar DataArray behaves the same as an integer.

Minimal Complete Verifiable Example

import xarray as xr

da = xr.DataArray([1, 2, 3], dims="x", coord={"k": ("x", [0, 1, 2])})
# <xarray.DataArray (x: 3)>
# array([1, 2, 3])
# Coordinates:
#     k        (x) int32 0 1 2

da.isel({"x": 1}, drop=True)
# works
# <xarray.DataArray ()>
# array(2)

da.isel({"x": xr.DataArray(1)}, drop=True)
# does not drop "k" coordinate
# <xarray.DataArray ()>
# array(2)
# Coordinates:
#     k        int32 1

Relevant log output

No response

Anything else we need to know?

No response

Environment

INSTALLED VERSIONS

commit: 4fbca23
python: 3.9.6 | packaged by conda-forge | (default, Jul 6 2021, 08:46:02) [MSC v.1916 64 bit (AMD64)]
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: AMD64 Family 23 Model 113 Stepping 0, AuthenticAMD
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: ('de_DE', 'cp1252')
libhdf5: 1.10.6
libnetcdf: 4.7.4

xarray: 0.18.2.dev76+g3a7e7ca2.d20210706
pandas: 1.3.0
numpy: 1.21.0
scipy: 1.7.0
netCDF4: 1.5.6
pydap: installed
h5netcdf: 0.11.0
h5py: 3.3.0
Nio: None
zarr: 2.8.3
cftime: 1.5.0
nc_time_axis: 1.3.1
PseudoNetCDF: installed
cfgrib: None
iris: 2.4.0
bottleneck: 1.3.2
dask: 2021.06.2
distributed: 2021.06.2
matplotlib: 3.4.2
cartopy: 0.19.0.post1
seaborn: 0.11.1
numbagg: 0.2.1
fsspec: 2021.06.1
cupy: None
pint: 0.17
sparse: 0.12.0
setuptools: 49.6.0.post20210108
pip: 21.3.1
conda: None
pytest: 6.2.4
IPython: None
sphinx: None

[headtr1ck]

It seems that this counts as "fancy indexing".
So either this intentional, or one could either add a check in Dataset._isel_fancy

if drop and new_var.ndim == 0:
    continue

or check if the indexer has ndim==0 in isel directly and do not enter the "fancy" path.

[benbovy]

Thanks for the report @headtr1ck, I see that it is a regression bug introduced in #5692 when refactoring _isel_fancy (drop is ignored now, it shouldn't).