State embedding not working depending on widget order in pn.Column

[bertcoerver]

ALL software version info

(this library, plus any other relevant software, e.g. bokeh, python, notebook, OS, browser, etc should be added within the dropdown below.)

Software Version Info

holoviews==1.22.1
panel==1.8.4
param==2.3.1
python==3.14.2

Description of expected behavior and the observed behavior

I'm exporting a pn.Column to html and embedding the different states using .save(..., embed=True). When I put one of the widgets (called model) at the end of my pn.Column, half of the saved states are "empty" (I can also see that by setting embed_json=True), while if I put the same widget at the top of my pn.Column everything works as expected.

Complete, minimal, self-contained example code that reproduces the issue

import holoviews as hv
import panel as pn
import numpy as np
import pandas as pd
import param
hv.extension("bokeh")

dfs = list()
for model in ["etlook","tsebtp"]:
    for class_ in [1,2,3,4,5]:
        values = np.random.randn(np.random.randint(40, 80))
        dfs.append(pd.DataFrame({
            "idx": np.arange(values.size),
            "count": values,
            "model": model,
            "class": class_
        }))
df = pd.concat(dfs).set_index("idx")

class Test(param.Parameterized):

    #####
    ## DATA
    #####
    df: pd.DataFrame = param.DataFrame(
    )

    #####
    ## WIDGETS
    #####
    a: int = param.Integer(
        default=0, 
        softbounds=(0,10), 
        metadata={"widget_type": pn.widgets.IntSlider},
    )
    fn: str = param.Selector(
        default="sin", 
        objects=["sin", "cos", "tan"],
        metadata={"widget_type": pn.widgets.RadioButtonGroup},
    )
    model: str = param.Selector(
        default="model1", 
        objects=["model1", "model2"],
        metadata={"widget_type": pn.widgets.RadioButtonGroup},
    )
    
    #####
    ## PANES
    #####
    curves: hv.DynamicMap = param.ClassSelector(
        class_=hv.DynamicMap, 
    )

    def __init__(self, **params):

        super().__init__(**params)

        self.curves = hv.DynamicMap(
            self.make_curve,
            streams={
                "a": self.param["a"],
                "fn": self.param["fn"],
            },
        ).opts(
            hv.opts.Curve(framewise=True, responsive=True),
        )

        self.view: pn.Column = self.make_view()

    def add_section(self, hv_name):
        out = list()
        if hv_name in self.param:
            out.append(pn.pane.HoloViews(getattr(self, hv_name)))
        else:
            raise ValueError
        return out
    
    def add_widget(self, param_name):
        return pn.Param(
                self.param[param_name], 
                     widgets = {param_name: self.param[param_name].metadata
                     }
                    )

    def make_curve(self, a, fn):
        x = np.linspace(0, 10, 100)
        fn_ = getattr(np, fn)
        y = fn_(x*a)
        return hv.Curve((x,y)).redim(x="the bins")
    
    def make_view(self):

        # NOTE this works.
        # view: pn.layout = pn.Column(
        #     self.add_widget("model"),
        #     *self.add_section("curves"),
        #     self.add_widget("a"),
        #     self.add_widget("fn"),
        # )

        # NOTE this does not work.
        view: pn.layout = pn.Column(
            *self.add_section("curves"),
            self.add_widget("a"),
            self.add_widget("fn"),
            self.add_widget("model"),
        )

        return view

if __name__ == "__main__":

    self = Test(
        df=df
    )

    print(self.view)

    if False:
        pn.serve(self.view)
    else:
        self.view.save(
            "only_works_when_model_is_model2.html", 
            embed=True, 
            max_opts=50, 
            embed_json=False
        )

Stack traceback and/or browser JavaScript console output

Screenshots or screencasts of the bug in action

Screen.Recording.2026-01-08.at.10.48.12.mov

only_works_when_model_is_model2.html
this_works.html

• I may be interested in making a pull request to address this