Getting the base html in the browser debugger

[blink1073]

Instead of the html for the JupyterLab page, I'm seeing the following:

Replication:

• Install jupyterlab from a git checkout
• Run pip install -e .
• Make sure you have [email protected]
• Run jupyter lab --dev-mode
• Open the browser console
• Refresh the page and go to the location shown above

$ pip list | grep jupyter
jupyter-client                6.1.6
jupyter-core                  4.6.3
jupyter-server                1.0.0rc6
jupyterlab                    3.0.0a10            /Users/stslve/workspace/jupyter/jlab-master
jupyterlab-server             2.0.0b1

This was in Firefox 68.8.0esr (64-bit)

[blink1073]

This does not happen every time, and I was not able to reproduce in Chrome 84.0.4147.125 (Official Build) (64-bit)