issues: 261805282

``` In [60]: da = xarray.DataArray([1, 2, 3], dims='x', coords={'y': ('x', [1, 1, 1])})

In [61]: da.groupby('y').apply(lambda x: x[:2])

IndexError Traceback (most recent call last) <ipython-input-61-4c28a4712c34> in <module>() ----> 1 da.groupby('y').apply(lambda x: x[:2])

~/dev/xarray/xarray/core/groupby.py in apply(self, func, shortcut, kwargs) 516 applied = (maybe_wrap_array(arr, func(arr, kwargs)) 517 for arr in grouped) --> 518 return self._combine(applied, shortcut=shortcut) 519 520 def _combine(self, applied, shortcut=False):

~/dev/xarray/xarray/core/groupby.py in _combine(self, applied, shortcut) 526 else: 527 combined = concat(applied, dim) --> 528 combined = _maybe_reorder(combined, dim, positions) 529 530 if isinstance(combined, type(self._obj)):

~/dev/xarray/xarray/core/groupby.py in _maybe_reorder(xarray_obj, dim, positions) 436 return xarray_obj 437 else: --> 438 return xarray_obj[{dim: order}] 439 440

~/dev/xarray/xarray/core/dataarray.py in getitem(self, key) 476 else: 477 # orthogonal array indexing --> 478 return self.isel(**self._item_key_to_dict(key)) 479 480 def setitem(self, key, value):

~/dev/xarray/xarray/core/dataarray.py in isel(self, drop, indexers) 710 DataArray.sel 711 """ --> 712 ds = self._to_temp_dataset().isel(drop=drop, indexers) 713 return self._from_temp_dataset(ds) 714

~/dev/xarray/xarray/core/dataset.py in isel(self, drop, indexers) 1172 for name, var in iteritems(self._variables): 1173 var_indexers = dict((k, v) for k, v in indexers if k in var.dims) -> 1174 new_var = var.isel(var_indexers) 1175 if not (drop and name in var_indexers): 1176 variables[name] = new_var

~/dev/xarray/xarray/core/variable.py in isel(self, **indexers) 596 if dim in indexers: 597 key[i] = indexers[dim] --> 598 return self[tuple(key)] 599 600 def squeeze(self, dim=None):

~/dev/xarray/xarray/core/variable.py in getitem(self, key) 426 dims = tuple(dim for k, dim in zip(key, self.dims) 427 if not isinstance(k, integer_types)) --> 428 values = self._indexable_data[key] 429 # orthogonal indexing should ensure the dimensionality is consistent 430 if hasattr(values, 'ndim'):

~/dev/xarray/xarray/core/indexing.py in getitem(self, key) 476 def getitem(self, key): 477 key = self._convert_key(key) --> 478 return self._ensure_ndarray(self.array[key]) 479 480 def setitem(self, key, value):

IndexError: index 2 is out of bounds for axis 1 with size 2 ```

This would be useful, for example, for grouped sampling: https://stackoverflow.com/questions/46498247/how-to-downsample-xarray-dataset-using-groupby

To fix this, we will need to update our heuristics that decide if a groupby operation is a "transform" type operation that should have the output reordered to the original order: https://github.com/pydata/xarray/blob/24643ecee2eab04d0f84c41715d753e829f448e6/xarray/core/groupby.py#L293-L299