Cannot control lazy-loading behavior on a featured image

[flixos90]

When lazy-loading was introduced in WordPress 5.5, one crucial customization point that was omitted (by accident) is checking whether lazy-loading is enabled for a featured image.

As first reported in ​https://github.com/WordPress/gutenberg/issues/46789, this means that it is impossible to control or remove loading="lazy" attributes added to featured images.

The Gutenberg issue also refers to other problems some of which are subject of #56930, but technically those problems are separate from this one.

Trac ticket: https://core.trac.wordpress.org/ticket/57490

[flixos90]

@adamsilverstein @joemcgill @mukesh27 Could you maybe take a look at this? The fix is pretty straightforward.

@joemcgill Fair point, it may be (or have been) a good idea to call wp_lazy_loading_enabled() in wp_get_loading_attr_default(), so that consuming code would have only needed to call the latter. However, the second function was only introduced later, and at this point I'm not sure we could make such a change without breaking BC.

[flixos90]

In 55093:

Media: Allow for customization of lazy-loading featured images.

When lazy-loading images was introduced, in [52065] the check for wp_lazy_loading_enabled() was omitted by accident in the logic to set the attribute with its default value on img tags from get_the_post_thumbnail(). Without this check, it is impossible for third-party developers to modify the behavior for featured images.

This changeset fixes the problem by introducing the check.

Props flixos90, joemcgill, mukesh27.
Fixes #57490.

Committed in https://core.trac.wordpress.org/changeset/55093.