Regression: JSComponent fails with AttributeError in Panel 1.6.1+ when using External Files for _esm and _stylesheets

[leighshepperson-gresearch]

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.)

Panel: 1.6.1
Bokeh: 3.2.0
Python: 3.10.4
Notebook: 6.5.4
OS: Ubuntu 22.04
Browser: Chrome 110

Description of expected behavior and the observed behavior

The JSComponent should be served without errors when using _esm and _stylesheets External Files, whether they are defined as Path objects or as strings, as it does in Panel 1.5.5.

Observed Behavior:
An AttributeError is raised in Panel >= 1.6.1 (including pre-release) when attempting to use lstrip on a PosixPath object in https://github.com/holoviz/panel/blob/main/panel/io/resources.py#L679, even when using string paths.

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

The example from the documentation has this issue https://panel.holoviz.org/reference/custom_components/JSComponent.html#external-files

from pathlib import Path
import param
import panel as pn
from panel.custom import JSComponent

pn.extension()

class CounterButton(JSComponent):
    value = param.Integer()

    # Using Path objects
    _esm = Path("counter_button.js")
    _stylesheets = [Path("counter_button.css")]

pn.serve(CounterButtonString())

# Alternatively, using string paths
class CounterButtonString(JSComponent):
    value = param.Integer()

    # Using string paths
    _esm = "counter_button.js"
    _stylesheets = ["counter_button.css"]

pn.serve(CounterButtonString())

Stack traceback and/or browser JavaScript console output

Traceback (most recent call last):
  File "/home/admin/PycharmProjects/PythonProject/.venv/lib/python3.10/site-packages/tornado/web.py", line 1790, in _execute
    result = await result
  File "/home/admin/PycharmProjects/PythonProject/.venv/lib/python3.10/site-packages/panel/io/server.py", line 524, in get
    page = server_html_page_for_session(
  File "/home/admin/PycharmProjects/PythonProject/.venv/lib/python3.10/site-packages/panel/io/server.py", line 295, in server_html_page_for_session
    bundle = bundle_resources(doc.roots, resources)
  File "/home/admin/PycharmProjects/PythonProject/.venv/lib/python3.10/site-packages/panel/io/resources.py", line 429, in bundle_resources
    css_files.extend(css_resources.css_files)
  File "/home/admin/PycharmProjects/PythonProject/.venv/lib/python3.10/site-packages/panel/io/resources.py", line 751, in css_files
    self.extra_resources(files, '_bundle_css')
  File "/home/admin/PycharmProjects/PythonProject/.venv/lib/python3.10/site-packages/panel/io/resources.py", line 679, in extra_resources
    if not isurl(resource) and not resource.lstrip('./').startswith('static/extensions'):
AttributeError: 'PosixPath' object has no attribute 'lstrip'
500 GET / (::1) 92.50ms

[philippjfr]

Good catch, thanks! Will fix.