Frontend updates aren't registered in IPywidgets

Updates to widgets (e.g. dragging the slider) trigger the comm message to be sent, and it's being received by python (the Comm.handle_msg is being called) but IPywidgets doesn't seem to actually be registering the change.

Start at the frontend update path and follow the comm message into the mentioned Comm.handle_msg entry point. Determine where the IPywidgets change registration stops; done means widget interactions such as dragging the slider are registered by IPywidgets after the message reaches Python.

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