fix indexing PandasMultiIndexingAdapater

When level is not None:

- if result is another adapter: propagate it properly
- if result is a numpy adapter: use level values

Author: benbovy

xarray/core/indexing.py

@@ -1483,7 +1483,7 @@ def __getitem__(
             (key,) = key
 
         if getattr(key, "ndim", 0) > 1:  # Return np-array if multidimensional
-            return NumpyIndexingAdapter(self.array.values)[indexer]
+            return NumpyIndexingAdapter(np.asarray(self))[indexer]
 
         result = self.array[key]
 
@@ -1542,6 +1542,13 @@ def _convert_scalar(self, item):
             item = item[idx]
         return super()._convert_scalar(item)
 
+    def __getitem__(self, indexer):
+        result = super().__getitem__(indexer)
+        if isinstance(result, type(self)):
+            result.level = self.level
+
+        return result
+
     def __repr__(self) -> str:
         if self.level is None:
             return super().__repr__()