[matthiasschabel]

Description

Extracting bare pixel samples from a raster with drop=True does not drop the scalar coordinates when the raster's index keeps them:

raster.isel(x=i, y=j, drop=True).coords     # still has 0-d 'x' and 'y'

• The documented contract of isel(..., drop=True) and sel(..., drop=True) is that scalar coordinates produced by the selection are dropped.
• An index whose isel rebuilds the selected position as a 0-d coordinate returns it in index_variables, and the isel paths copy every index variable into the result before drop is considered.
• The built-in indexes return None from isel for a scalar selection, so they never hit this and are unchanged.

The fix adds drop_scalar_index_coords, called from DataArray.isel, Dataset.isel and Dataset._isel_fancy when drop=True. It drops the index-owned scalar coordinates the way drop_vars would and raises the existing corrupt-index error when only some of an index's coordinates would go (selecting one axis of the raster with drop=True). Indexes the selection does not touch are left alone; sel(drop=True) delegates to isel.

On the new error: the previous result broke the drop=True contract, and drop_vars already raises the same error for a partial drop. It only affects custom indexes that keep scalar coordinates, so it is listed under Bug Fixes.

Reproducer with a scalar-keeping RasterIndex

The guide's RasterIndex plus an isel that keeps the selected position as a 0-d coordinate.

import numpy as np, xarray as xr
from xarray import Index, Variable
from xarray.core.indexes import PandasIndex

class RasterIndex(Index):
    def __init__(self, xy_indexes, scalars=None):
        self._xy_indexes = dict(xy_indexes); self._scalars = dict(scalars or {})
    @classmethod
    def from_variables(cls, variables, *, options):
        return cls({k: PandasIndex.from_variables({k: v}, options=options) for k, v in variables.items()})
    def create_variables(self, variables=None):
        out = dict(self._scalars)
        for index in self._xy_indexes.values():
            out.update(index.create_variables(variables))
        return out
    def isel(self, indexers):
        axes, scalars = {}, dict(self._scalars)
        for k, idx in self._xy_indexes.items():
            if idx.dim in indexers:
                new = idx.isel({idx.dim: indexers[idx.dim]})
                if new is None: scalars[k] = Variable((), idx.index[indexers[idx.dim]])
                else: axes[k] = new
            else: axes[k] = idx
        return RasterIndex(axes, scalars)

da = xr.DataArray(np.zeros((100, 50)), coords={"x": np.arange(50), "y": np.arange(100)}, dims=("y", "x"))
raster = da.set_xindex(["x", "y"], RasterIndex)
print(raster.isel(x=1, y=0, drop=True).coords)   # main: x and y kept; this PR: empty

Checklist

• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
    Tools: Codex (gpt-6-sol) drafted the change and tests at my direction; Claude (Opus 5.5) and Codex (gpt-6-astra) reviewed. I have read and validated every line.