It might be helpful to assert specifically that the natural present has not been overriden by the Theme version from block-theme's theme.json.

The original bug related to the theme presets with the same slug overriding the core presets so it might be good to specifically assert to focus the test on this aspect.

Maybe also include a reference to the location of the related core preset?

This value would never be overridden. What would happen instead is that theme array would have the natural preset. You may want to check by setting shadow.defaultPresets:false in core's theme.json and then execute npm run test:php -- --filter test_theme_shadow_presets_do_not_override_default_shadow_presets.

Note that I've also added a theme preset that is actually added (test) to make sure this is also working.

Essentially, presets work this way:

1. Once the different theme.json sources (default, theme, user) are merged, all the values are available under the source key. For example:

array(
  'default' => array( /* come from core's theme.json */
    array( 'slug' => 'natural', /* ...*/ ),
    array( 'slug' => 'deep', /* ...*/ ),
  ),
  'theme'  => array( /* come from the theme's theme.json */
    array( 'slug' => 'natural', /* ...*/ ),
    array( 'slug' => 'test', /* ...*/ ),
  )
)

2. However, in certain situations, we don't want the theme presets to "override" default presets, so we remove any whose slug is the same as one of the default presets. This is what happens for shadows, resulting in the following:

array(
  'default' => array( /* come from core's theme.json */
    array( 'slug' => 'natural', /* ...*/ ),
    array( 'slug' => 'deep', /* ...*/ ),
  ),
  'theme'  => array( /* come from theme's theme.json, after removing those whose slug is the same as any of the default presets */
    array( 'slug' => 'test', /* ...*/ ),
),
)

This is the base behavior for all presets. From there, this data is used differently by the components:

• UI: some design tools may want list the presets from source (this is what colors do) but others display them all together (font families).
• CSS: in general, every preset item available in the default and theme array will be converted to --wp--preset--category--slug (e.g.: --wp--preset--shadow-natural). This was the issue Alex raised and that is fixed by this PR.

Thank you for the additional clarification. I was understanding correctly, but perhaps not communicating that understanding very well.

When I say "overridden" I mean that the bug was that the Theme preset would take precedence over the Core preset.

Now with this PR the Core preset takes precedence over the Theme preset.

The condition is that they must have a matching slug. This is why you've added a "Test" preset that only exists in the Theme to verify that this functionality still works.