Only load Jupyter JS dependencies on pages that need it

[choldgraf]

Description / Summary

I did a quick lighthouse audit on Jupyter Book, and discovered that a large chunk of our Javascript loading is coming from loading the JS bundles needed for Jupyter widgets (there are some other improvements we can make too, will open other issues for that):

This is something we could load in a more restrictive way. Here are a few ideas from anticipated easiest-to-hardest:

Load this JS bundle only if:

• The page is an ipynb file
• The page is known to have notebook content (e.g., also covering MyST Markdown Notebooks)
• The page is known to have widget content on it (maybe as part of the CellOutputBundle parsing logic?)

Value / benefit

This could save a lot of unnecessary JS loading, and speed up page load times.

Implementation details

I think these are the two libraries we could target:

• <script src="https://cdnjs.cloudflare.com/ajax/libs/require.js/2.3.4/require.min.js"></script>
• <script src="https://unpkg.com/@jupyter-widgets/html-manager@^0.20.0/dist/embed-amd.js"></script>

somewhere around here:

MyST-NB/myst_nb/__init__.py

Lines 284 to 301 in 2cc1260

	require_url_default = (
	REQUIRE_URL_DEFAULT
	if "jupyter_sphinx_require_url" not in app.config
	else app.config.jupyter_sphinx_require_url
	)
	embed_url_default = (
	None
	if "jupyter_sphinx_embed_url" not in app.config
	else app.config.jupyter_sphinx_embed_url
	)
	if require_url_default:
	builder.add_js_file(require_url_default)
	embed_url = embed_url_default or DEFAULT_EMBED_REQUIREJS_URL
	else:
	embed_url = embed_url_default or DEFAULT_EMBED_SCRIPT_URL
	if embed_url:
	builder.add_js_file(embed_url)

Tasks to complete

No response

[akhmerov]

Switching to a single page app would also diminish the impact of the problem.

[chrisjsewell]

This is available since sphinx 3.5: sphinx-doc/sphinx#8631

[choldgraf]

A single page app would be a pretty substantial change. I've never seen a Sphinx site served as a single page app. Though you could probably do it with something like turbolinks. Maybe something to explore after we've picked off the lower hanging fruit.

@chrisjsewell agree that sounds like the right mechanism to use. Id be fine pinning our minimum sphinx 3 version to take advantage is this. You see any problems with that?

[chrisjsewell]

You can just change the logic based on the sphinx version, ps don't worry about a PR, I'll add it into https://github.com/executablebooks/MyST-NB/tree/refactor-mystnb, which is on its way

[chrisjsewell]

Now fixed in #380, although feel free to double-check @choldgraf