Clarify `wp_should_load_separate_core_block_assets()`, `block.json` script expectations

[jeremyfelt]

In the block.json ​schema, the script property is described as:

"Block type frontend and editor script definition. It will be enqueued both in the editor and when viewing the content on the front of the site."

From that description, I expect the script set in my block's block.json to load in the editor and on the front-end when my block is in use. I do not expect the script to load on the front-end when my block is not in use.

If I activate TwentyTwentyTwo, everything works as expected.

If I activate TwentyTwentyOne, my script appears on every front-end view, whether or not the block is loaded.

The cause of this can be tracked to wp_should_load_separate_core_block_assets(), which has a short description of:

Checks whether separate styles should be loaded for core blocks on-render

This implies at the beginning of the function documentation that the function will only apply to core blocks.

The longer description helps a bit, but is muddied up in talking about core and third-party blocks separately when in reality it seems that they are treated the same. It mentions block-library/style.css, which itself implies this _might_ be talking about styles rather than all assets.

The filter inside the function, should_load_separate_core_block_assets, defaults to false for themes that are not FSE and has a short description of:

Filters whether block styles should be loaded separately.

The longer description refers only to "core block assets".

---

Proposal

My guess is that we can't just change the filter to default to true, though that would be ideal. :)

I think the function docs could be clarified with something like:

/**
 * Checks whether block assets should always load or only on-render.
 *
 * When this function returns true, block assets defined by the `script` and
 * `style` properties in `block.json` are loaded on-render.
 *
 * When this function returns false, these block assets are always loaded,
 * regardless of whether the block is rendered in a page.
 *
 * This only affects front end and not the block editor screens.

And the filter docs with something like:

 * Filters whether block assets should always load or load only on-render.
 *
 * Returning false loads all block assets, regardless of whether they are rendered
 * in a page or not. Returning true loads block assets only when they are rendered.

Previously #53505

[pentatonicfunk]

This filter should be default true IMO, i dont see any reason why non block theme want it to be false by default, most of the times the block styles/scripts only needed when its need to be rendered.
If theme ( or plugin ) needs the block styles/scripts when the block isn't rendered, it most likely doing it wrong.