Restrict "Default Post Format" Options to Formats Supported by the Active Theme

[sahil-gidwani]

This PR updates the options-writing.php admin screen to ensure that the "Default Post Format" dropdown only displays post formats explicitly supported by the currently active theme. Previously, this dropdown listed all core post formats, regardless of whether the theme declared support via add_theme_support( 'post-formats', [...] ).

This behavior led to possible misconfiguration, user confusion, and inconsistent behavior between the settings screen and the post editor (which already hides unsupported formats).

Previous Logic

The dropdown was populated using:

$post_formats = get_post_format_strings();
unset( $post_formats['standard'] );

This included all formats regardless of theme support.

New Logic

Replaced with:

$supported_formats = get_theme_support( 'post-formats' );

if ( is_array( $supported_formats ) && isset( $supported_formats[0] ) ) {
    $post_formats = array_intersect_key( get_post_format_strings(), array_flip( $supported_formats[0] ) );
} else {
    $post_formats = array();
}

This ensures only the intersection of supported formats and registered format strings is used. The standard format remains available separately, as it is treated as the fallback/default.

Why This Matters

• Prevents the admin UI from offering unsupported options.
• Aligns admin settings with front-end and post editor behavior.
• Avoids storing invalid default values in default_post_format which have no effect on post rendering.
• Reduces user confusion by not listing formats that won’t have any visible impact.

Edge Cases Handled

• If the theme supports no formats, only the standard option is shown.
• If the theme supports all formats, the dropdown behavior remains unchanged.
• Backward compatibility is preserved, as the underlying storage and post format API remains the same.

Trac Link: https://core.trac.wordpress.org/ticket/43613

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