Fix issue #283, add dns-prefetch, preconnect for editor iframe

[schalkneethling]

Adds dns-prefetch and preconnect for the editor iframe to test the performance impact.

[codecov-io]

Codecov Report

Merging #4455 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4455   +/-   ##
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  Coverage   94.71%   94.71%           
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  Files         260      260           
  Lines       22805    22805           
  Branches     1674     1674           
=======================================
  Hits        21600    21600           
  Misses        981      981           
  Partials      224      224

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[schalkneethling]

I felt confident that this is the correct file but, now I am not so sure. When I look at a page such as http://127.0.0.1:8000/en-US/docs/Experiment:InteractiveEditor/Array.prototype.reduce() I do not see the link elements in the header :(

[jwhitlock]

That's because it is overridden in document.html, which is a better place for this code:

https://github.com/mozilla/kuma/blob/a608faaa7c190b094fe15eda33877050e34ecb95/kuma/wiki/jinja2/wiki/document.html#L66-L86

[schalkneethling]

Thanks @jwhitlock Will update the PR

[stephaniehobson]

Do we think there are any downsides to adding it to all pages?

I think it is possible to include the code only if the interactive examples are on the page. We do some custom configuration on the kuma side in response to the existence of the quick-links class for left side menus. (I'm not sure enough about how it works to give instructions though).

[schalkneethling]

I think it is possible to include the code only if the interactive examples are on the page.

@stephaniehobson That would be great, as long as it is not injected with JS. I cannot find the doc now, but I remember recently hearing something about Chrome not behaving predictably if you inject these tags with JS. I will try and find the reference to it. Perhaps it is under x condition which does not apply here.

[schalkneethling]

@stephaniehobson The seems to be what you are referring to? https://github.com/mozilla/kuma/blob/master/kuma/wiki/jinja2/wiki/document.html#L316

and

https://github.com/mozilla/kuma/blob/master/kuma/wiki/jinja2/wiki/document.html#L28

and

https://github.com/mozilla/kuma/blob/master/kuma/wiki/views/document.py#L698

and

https://github.com/mozilla/kuma/blob/master/kuma/wiki/views/document.py#L722

[stephaniehobson]

Yes! That's it exactly. Great detective work :)

[stephaniehobson]

Good now that John's changes are integrated. My comments can be acted on if we see performance problems.