JupyterLab custom extension won't load

[b3m2a1]

Does this issue occur when all extensions are disabled?: Yes/No

• VS Code Version:
• OS Version:

Steps to Reproduce:

I have a JupyterLab extension that is installed when I run pip install, this doesn't work with the Jupyter Extension for VS Code for some reason.

Here's what it looks like in JupyterLab

Here's what I get in VS Code

It is failing to load the TS from the appropriate location. It is not documented what Jupyter runtime the notebook system is even using, so I can't debug this for myself

[Yonas650]

Likely cause & proposal

Thanks for the report! In VS Code, notebooks don’t load JupyterLab front-end (lab) extensions. Only kernel/server-side Python code affects execution. So JS/TS that works in JupyterLab’s UI won’t run inside VS Code’s notebook UI unless there’s a matching VS Code notebook renderer (or it uses supported ipywidgets/MIME).
Could you share:

• The package name you pip install (and whether it includes a labextension vs a server extension).

• Any custom MIME types it outputs (e.g., application/vnd.myext+json).

• Logs: Jupyter: Show Output and Developer: Toggle Developer Tools console.

If this is a labextension, I can:
1. open a small PR to improve the Jupyter extension docs/error message (clarifying that JupyterLab UI extensions aren’t loaded; suggest renderer/widgets), and
2. prototype a minimal VS Code notebook renderer for the package’s MIME so its outputs display in VS Code.

[b3m2a1]

The package is mccoygroup-mcutils, it includes a vendored version of this extension package: https://github.com/b3m2a1/ActiveHTMLWidget

The package is installed by default with the following setup.py block

    data_files=[
        ('share/jupyter/nbextensions/ActiveHTMLWidget', glob.glob('McUtils/Jupyter/JHTML/ActiveHTMLWidget/nbextension/*')),
        ('share/jupyter/labextensions/ActiveHTMLWidget/static', glob.glob('McUtils/Jupyter/JHTML/ActiveHTMLWidget/labextension/static/*.js')),
        ('share/jupyter/labextensions/ActiveHTMLWidget', glob.glob('McUtils/Jupyter/JHTML/ActiveHTMLWidget/labextension/*.json')),
        ('share/jupyter/labextensions/ActiveHTMLWidget', ['McUtils/Jupyter/JHTML/ActiveHTMLWidget/install.json']),
        ('etc/jupyter/nbconfig/notebook.d', ['McUtils/Jupyter/JHTML/ActiveHTMLWidget/ActiveHTMLWidget.json']),
    ]

Both the lab extension and nbextension work

As an update, I just tried this again this morning and the VS Code appeared to work, at least the HTML/CSS for my widget was showing, not sure if the widget interactions work, but nothing I was doing yesterday could get it to work.

VS Code also removed the ability to select any of the Jupyter kernels I'd enabled in the Jupyter > Kernels > Trusted dialog although it (temporarily) remembered the Python Environment executable from my Jupyter kernel venv from which I'm assuming it loaded the nbextension JS. After switching to a different Python Environment it has lost that and now nothing I try will get any of my kernels to load again, meaning I am unable to test further for the moment