[wp-trac] [WordPress Trac] #58870: Delay loading comment-reply script with async loading strategy

[WordPress Trac]

#58870: Delay loading comment-reply script with async loading strategy
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 Reporter:  westonruter              |      Owner:  (none)
     Type:  enhancement              |     Status:  new
 Priority:  normal                   |  Milestone:  6.4
Component:  Comments                 |    Version:  trunk
 Severity:  normal                   |   Keywords:
  Focuses:  javascript, performance  |
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 Currently the `comment-reply` script is loaded in the footer. This is
 great because it doesn't block rendering, however it can still block other
 scripts from executing and thus it delays `DOMContentLoaded`. This means
 that if a theme has a script with logic running at `DOMContentLoaded` to
 add interactivity to the nav menu, this will get delayed for the sake of
 `comment-reply` which is not even needed unless the user scrolls down the
 page to the comments. In the worst case scenario, a user may try to tap on
 the nav menu too early and nothing may happen due to `comment-reply`
 causing an unnecessary delay.

 Adding either `defer` or `async` to the `comment-reply` script will cause
 the browser to treat it as low priority from a [https://web.dev/fetch-
 priority/#browser-priority-and-fetchpriority loading perspective].
 However, in the case of `defer` it will still block other `defer` scripts
 and the `DOMContentLoaded` event. With `async` there is the possibility
 that the execution may be delayed until after the DOM loads if the script
 is not in the cache yet. On the other hand, if the `async` script is
 cached, it may execute right away which would be no different than the
 current situation of it being a blocking script in the footer: so no
 better or worse. But for the case where the `comment-reply` script is not
 cached yet, `async` would surely be better.

 Additionally, according to [https://www.tunetheweb.com/about/ Barry
 Pollard] there is talk of executing all `defer` scripts together in one
 single task rather than running each in their own task. Using `async`
 would prevent adding another script to this `defer` group.

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Ticket URL: <https://core.trac.wordpress.org/ticket/58870>
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