Defer JavaScript scripts in html_document

[atusy]

This PR defers JavaScript scripts that are currently called as DOMContentLoaded event handlers.

Although this works fine in static html_document, it still fails in runtime: shiny.
This is probably because src is converted wrongly.
See highlighted line and the last line in the following image.

Reprex

---
title: "My First R Markdown Document" 
output:
  html_document:
    anchor_sections: true
    self_contained: false
runtime: shiny
---

# fooo

(closes #2032)

[yihui]

This is probably because src is converted wrongly.

<script src="header-attrs-2.6.6/%5Bobject%20Object%5D"></script>

is this when decoded:

<script src="header-attrs-2.6.6/[object Object]"></script>

It seems when the defer attribute is present, the src value is calculated by adding the string "header-attrs-..." to an object like

> "header-attrs-.../" + { "src": "foo.js", "defer": null}

"header-attrs-.../[object Object]"

I don't know where this string concatenation occurs. We will need help from the Shiny team (perhaps @cpsievert).

[cpsievert]

I'm pretty sure the script rendering issue is essentially a duplicate of rstudio/shiny#3345

I'll try to have a fix up by the end of the week

[cpsievert]

Ok, rstudio/shiny#3345 should be fixed on the main branch now, so the <script defer> approach shouldn't be a blocker anymore.

By the way, it's not immediately obvious to me that defer is the best way to solve this issue -- you might want to consider using setTimeout() instead since that's closer to what we do elsewhere to address these sort of timing issues

[atusy]

@cpsievert Thanks a lot!

setTimeout()

Would you tell me some background of this suggestion?
I think it is quite difficult to predict the adequate value for it.

[atusy]

Maybe the best way is to

1. check if document.readyState is complete
   • this is what happens by runtime: shiny
2. if true, run function immediately or use setTimeout
3. otherwise, addEventListener as usual.

I wonder if there is an easy way to handle this for developers of custom formats or HTML dependencies.
It would be nice if they even do not have to think about it.
Maybe defer is not the best, but the quickest way for them.