I've many, many times been wanting to create custom widgets using the Viewer. But I've experienced just as many times its not possible.

For example now to help P720 in https://discourse.holoviz.org/t/panel-chat-interface/7505 I would like to create a custom widget and use that for the ChatInterface. But it cannot work as the Viewer cannot be made to work like a Widget.

Minimum, Reproducible Example

import panel as pn
import param

pn.extension()

class PreferenceInput(pn.viewable.Layoutable, pn.widgets.WidgetBase, pn.viewable.Viewer):
    value = param.Parameter()

    def __init__(self, **params):
        super().__init__(**params)
        
        self._preference_widget = pn.widgets.RadioBoxGroup(options=["blue", "red", "green"], value="blue", name="preference")
        pn.bind(self._update_value, self._preference_widget)
        self._layout = pn.Column(
            "What is your preference?",
            self._preference_widget,
        )
    
    def _update_value(self, preference):
        self.value = "My preference is {preference}."

    def __panel__(self):
        return self._layout

preference_input = PreferenceInput()

def even_or_odd(contents, user, instance):
    if len(contents) % 2 == 0:
        return "Even number of characters."
    return "Odd number of characters."

pn.chat.ChatInterface(callback=even_or_odd, widgets=[preference_input]).servable()

It works fine until I click send. Then I get

  File "/home/jovyan/repos/private/panel/panel/chat/interface.py", line 405, in _click_send
    value = active_widget.value

In this case the problem is that when a Viewer is put in a row its replaced with its __panel__(). Thus then the assumptions about how to retrieve the ChatInterface.active_widget property breaks down. In this case you could probable "fix" the active_widget implementation. But in my experience its a problem all over the place that its not possible to get a Viewer to behave like a widget.

As a bonus its also hard and not documented how to get a Viewer to be Layoutable. That is always another friction. The ChatInterface also assumes that about the input widgets. It assumes there is a sizing_mode on the widget.