add warning when a ref sidecar has anchor='right'

Author: bmorris3

sidecar/sidecar.py

@@ -7,9 +7,9 @@
 """
 TODO: Add module docstring
 """
-
+import warnings
 from ipywidgets import Output, widget_serialization
-from traitlets import Unicode, CaselessStrEnum, Any, Instance
+from traitlets import Unicode, CaselessStrEnum, Any, Instance, observe
 from ._frontend import EXTENSION_SPEC_VERSION
 
 module_name = "@jupyter-widgets/jupyterlab-sidecar"
@@ -30,3 +30,11 @@ class Sidecar(Output):
     ).tag(sync=True)
     ref = Instance('sidecar.Sidecar', allow_none=True).tag(sync=True, **widget_serialization)
     _widget_id = Any().tag(sync=True)
+
+    @observe('ref')
+    def _validate_ref_anchor(self, *args):
+        if self.ref.anchor == 'right':
+            warnings.warn(
+                "`ref` cannot be set when `ref.anchor == 'right'`. "
+                "Proceeding with `ref = None`.", UserWarning
+            )