API: reimplement FixedWindowIndexer.get_window_bounds to fix groupby bug

pandas/core/window/common.py

@@ -70,6 +70,7 @@ def _apply(
         floor: int = 1,
         is_weighted: bool = False,
         name: Optional[str] = None,
+        func_type: Optional[str] = None,
         use_numba_cache: bool = False,
         **kwargs,
     ):

pandas/core/window/indexers.py

@@ -78,17 +78,17 @@ def get_window_bounds(
         closed: Optional[str] = None,
     ) -> Tuple[np.ndarray, np.ndarray]:
 
-        start_s = np.zeros(self.window_size, dtype="int64")
-        start_e = (
-            np.arange(self.window_size, num_values, dtype="int64")
-            - self.window_size
-            + 1
-        )
-        start = np.concatenate([start_s, start_e])[:num_values]
+        if center:
+            offset = (self.window_size - 1) // 2
+        else:
+            offset = 0
+
+        end = np.arange(1 + offset, num_values + 1 + offset).astype("int64")
+        start = end - self.window_size
+
+        end = np.clip(end, 0, num_values)
+        start = np.clip(start, 0, num_values)
 
-        end_s = np.arange(self.window_size, dtype="int64") + 1
-        end_e = start_e + self.window_size
-        end = np.concatenate([end_s, end_e])[:num_values]
         return start, end