[robelsust]

Trac ticket: https://core.trac.wordpress.org/ticket/65979

Description

Restores the install_themes_tabs filter in wp-admin/theme-install.php to allow registering, reordering, and filtering tabs in the Backbone-based theme installer, matching parity with install_plugins_tabs on the Add Plugins screen.

Background & Architecture

• Prior to WordPress 3.8/3.9, the Add Themes screen was rendered via WP_Theme_Install_List_Table, which used the install_themes_tabs filter (introduced in 2.8.0).
• When theme-install.php transitioned to a Backbone.js single-page application, the filter links (<ul class="filter-links">) were hardcoded to 'popular', 'new', 'block-themes', and 'favorites'. The DocBlock for install_themes_tabs was demoted to "for backward compatibility only, for the suppression of the upload tab".
• Consequently, hosts, managed platforms, and custom repository plugins were forced into fragile DOM manipulation (jQuery('.filter-links').append(...)) to add custom tabs.
• The server-side AJAX query infrastructure in wp-admin/includes/ajax-actions.php (wp_ajax_query_themes()) is already fully dynamic: any browse key automatically applies install_themes_table_api_args_{$browse} and passes $args directly to themes_api( 'query_themes', $args ).

Changes Made

1. wp-admin/theme-install.php:

   • Defines the core tabs (popular, new, block-themes, favorites, and conditionally upload when the user has upload_themes capability) and runs apply_filters( 'install_themes_tabs', $tabs ) before wp_localize_script().
   • Computes $browse_tabs = array_diff_key( $tabs, array( 'upload' => true ) ); and derives $default_tab = ! empty( $browse_tabs ) ? (string) array_key_first( $browse_tabs ) : 'popular';.
   • Passes $default_tab to _wpThemeSettings.settings.defaultTab.
   • Renders the <ul class="filter-links"> list dynamically from $browse_tabs.
   • Preserves upload tab suppression via ! empty( $tabs['upload'] ) && current_user_can( 'upload_themes' ).
   • Parses both $_REQUEST['browse'] and $_REQUEST['tab'] for $tab.

2. src/js/_enqueues/wp/theme.js:

   • Updates InstallerRouter and RunInstaller.init() to route to _wpThemeSettings.settings.defaultTab || 'popular' when visiting the base URL without query parameters.
   • Adds route 'theme-install.php?tab=:sort': 'sort' to support legacy ?tab= URLs alongside ?browse=.
   • Wraps the sort tab selector in $.escapeSelector( sort ) to safely handle custom tab keys with special characters.

3. tests/phpunit/tests/theme/installThemesTabs.php:

   • Adds dedicated unit tests covering:
     • Default tabs for users with upload_themes capability.
     • Default tabs for users without upload_themes capability.
     • Registering custom tabs via install_themes_tabs.
     • Suppressing the upload tab via unset( $tabs['upload'] ).
     • Reordering tabs / changing the default active tab.
     • Fallback when browse tabs are empty.

Testing Instructions

1. Run PHPUnit tests:

   vendor/bin/phpunit tests/phpunit/tests/theme/installThemesTabs.php

2. In a test plugin or functions.php, add a custom tab:

   add_filter( 'install_themes_tabs', function( $tabs ) {
       $tabs['featured'] = 'Featured';
       return $tabs;
   } );

3. Visit wp-admin/theme-install.php.
4. Verify the "Featured" tab appears in the navigation links and clicking it navigates to ?browse=featured without errors.
5. Filter args via install_themes_table_api_args_featured or themes_api and verify themes render correctly.

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