Large list of spacing sizes causes var:preset|spacing|default value for other dimensions

[wongjn]

Description

If a theme has more than 8 spacing size presets, when customizing a dimension for margin, padding or block gap in the block editor for a block, the other non-customized dimensions for that "dimension group" will have a value of var:preset|spacing|default which for most themes will not exist.

8 is the threshold for whether <SpacingInputControl> displays a range control or a dropdown control:

https://github.com/WordPress/gutenberg/blob/v16.1.2/packages/block-editor/src/components/spacing-sizes-control/input-controls/spacing-input-control.js#L60

It seems likely then that it could be a bug with the way the dropdown control is used to handle the default value.

Step-by-step reproduction instructions

1. Edit wp-content/themes/twentytwentythree/theme.json to add 3 extra members in settings.spacing.spacingSizes:

   		"slug": "80",
   		"name": "6"
   +	},
   +	{
   +		"size": "1rem",
   +		"slug": "foo",
   +		"name": "foo"
   +	},
   +	{
   +		"size": "2rem",
   +		"slug": "bar",
   +		"name": "bar"
   +	},
   +	{
   +		"size": "3rem",
   +		"slug": "baz",
   +		"name": "baz"
    	}
    ],
    "units": [
    	"%",

2. Enable Twenty Twenty-Three theme.
3. Go into the Site Editor and edit the Footer template part.
4. Edit the top padding of the top level core/group block.
5. See that the left and right dimensions now lose their global padding and instead have the invalid var:preset|spacing|default value.

Screenshots, screen recording, code snippet

✅ With 8 or less spacing sizes

The other padding dimensions are not set when editing the top padding (commented formatted for readability):

<!-- wp:group {
  "style":{
    "spacing":{
      "padding":{
        "top":"var:preset|spacing|40"
      }
    }
  },
  "layout":{"type":"constrained"}
} -->

❌ With more than 9 spacing sizes

The other padding dimensions are set to var:preset|spacing|default when editing the top padding (commented formatted for readability):

<!-- wp:group {
  "style":{
    "spacing":{
      "padding":{
        "top":"var:preset|spacing|40",
        "right":"var:preset|spacing|default",
        "bottom":"var:preset|spacing|default",
        "left":"var:preset|spacing|default"
      }
    }
  },
  "layout":{"type":"constrained"}
} -->

Environment info

WordPress 6.3-RC2-56310

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes