Lighthouse: False positive blocking scripts

[EnoahNetzach]

Scripts loaded after the main load event are considered as blocking if they are prefetched in the head.

It is exemplified in this gist, where in the document's head a

<link as="script" src="/main.js" rel="preload">

tag is present and at the end of the body the preloaded script is added by

document.createElement('script')
script.src = '/main.js'
document.head.appendChild(script)

(Opened after https://github.com/google/WebFundamentals/issues/4694)

[patrickhulce]

Hm we didn't do the same handling for scripts that we did for links since defer and async make it a much less common use case. What's the primary benefit of this method over marking as async?

[EnoahNetzach]

What's the primary benefit of this method over marking as async?

It's that I can chain multiple script loads, having control over the request order.

[patrickhulce]

I see, and the script elements need to be appended to the head, not the body?

[EnoahNetzach]

That's not important, in facts the fastest solution was to change that.
I'm opening this issue just to notify it 😉

[patrickhulce]

gotcha thanks for the report! :)

[paulirish]

We think isLinkPreload from here https://chromedevtools.github.io/devtools-protocol/tot/Network/#type-Request will sort this out... But also it's implemented like this already. TBD!

[mixed]

Hi. There. I think that This issue is DONE.
https://github.com/GoogleChrome/lighthouse/blob/master/lighthouse-core/gather/gatherers/dobetterweb/tags-blocking-first-paint.js#L130