JS callback in button.js_on_click is runned twice when using custom templates

[rafgonsi]

ALL software version info

panel 0.10.0, bokeh 2.2.2, Python 3.7
OS: Manjaro 20.1.2
Browsers: Firefox 82.0-0.1, Google chrome 86.0.4240.111-1 (issue observed on both)

Description of expected behavior and the observed behavior

The problem appears only when using custom templates. When clicking the button with defined js-callback, the callback is executed twice. In this example I try to open a new tab. When it happens for the first time, browser opens new tab and asks me for permission to display pop-ups. If I agree, then in the future clicking the button will open two tabs. See gif below.

When specifying no template, everything works as expected.

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

import panel as pn

button = pn.widgets.Button(name="Button")
button.js_on_click(code='window.open("https://www.google.com/")')
tmpl = pn.Template("{% extends base %}")
tmpl.add_panel("some_name", button)
tmpl.servable()  # This opens new tab twice

button.servable()  # This works fine

Above code is run with

panel serve bug_example.py

Screenshots or screencasts of the bug in action

Workaround

One can use bokeh buttons to avoid this issue:

from bokeh.models import Button, CustomJS
from bokeh import events

bokeh_button = Button()
callback = CustomJS(code='window.open("https://www.google.com/")')
bokeh_button.js_on_event(events.ButtonClick, callback)
tmpl_bokeh = pn.Template("{% extends base %}")
tmpl_bokeh.add_panel("some_name", bokeh_button)
tmpl_bokeh.servable()