Themes: Show Preview button for block themes on Add Themes screen

[sanketio]

Description

After installing a block theme from Appearance → Themes → Add New Theme, the "Preview" button was missing — only "Activate" appeared. Classic themes correctly showed both "Activate" and "Live Preview". This inconsistency was present across all tabs and search results.

The root cause is threefold:

1. wp_ajax_install_theme() was always sending customizeUrl pointing to the classic Customizer (customize.php) for all themes, including block themes. Block themes don't support the Customizer, so the button was either broken or absent.

2. wp_ajax_query_themes() (the on-load data for all theme cards) was sending site-editor.php for installed block themes, but without the wp_theme_preview=<slug> query parameter. Without it, clicking Preview would open the Site Editor for the currently active theme instead of the selected one.

3. The Underscore.js template in theme-install.php had a blanket <# if ( ! data.block_theme ) { #> guard added by #54878 in 2022 to prevent pointing block themes at the Customizer. That suppression was never updated after context-aware Site Editor preview URLs became available.

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Changes

wp-admin/includes/ajax-actions.php — two functions:

• wp_ajax_install_theme(): For block themes, sends site-editor.php?wp_theme_preview=<slug>&return=... instead of the classic Customizer URL. Mirrors the existing logic in class-theme-installer-skin.php used by the non-AJAX full-page install path. Capability check also broadened to allow block themes regardless of the customize cap.
• wp_ajax_query_themes(): Adds wp_theme_preview=<slug> to the Site Editor URL for already-installed block themes shown on page load, so the button actually previews the correct theme.

wp-admin/theme-install.php:

• Replaces the blanket block-theme suppression with an explicit branch: block themes render a plain Preview link (no load-customize); classic themes keep Live Preview with load-customize.

js/_enqueues/wp/updates.js:

• installThemeSuccess(): Checks response.blockTheme when replacing the .preview button post-install. Block themes get a plain Preview <a> without the load-customize class (which triggers Customizer-specific JS); classic themes are unchanged.

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Testing Instructions

Prerequisites: A WordPress install with at least one classic theme and one block theme available but not yet installed.

Block theme — AJAX install path

1. Go to Appearance → Themes → Add New Theme.
2. Search for a block theme (e.g. "Twenty Twenty-Four") or open the Block Themes tab.
3. Click Install on any block theme that is not yet installed.
4. Wait for installation to complete.
5. Both Activate and Preview buttons should appear immediately.
6. Click Preview — it should open the Site Editor with that theme previewed, not the Customizer.

Classic theme — AJAX install path

1. On the same screen, install a classic theme (e.g. "Twenty Twenty").
2. Both Activate and Live Preview buttons should appear.
3. Click Live Preview — it should open the Customizer with that theme.

Already-installed block theme cards (page-load path)

1. Install a block theme via any method and do not activate it.
2. Return to Appearance → Themes → Add New Theme and browse any tab.
3. Find the installed block theme card.
4. The card should show Activate and Preview.
5. Click Preview — it should open the Site Editor previewing that specific theme, not the currently active one.

No regression — full-page install path

1. Navigate directly to wp-admin/update.php?action=install-theme&theme=<slug> (non-AJAX install).
2. Post-install buttons remain correct — this path was already working via class-theme-installer-skin.php.

Active block theme

1. Activate a block theme.
2. Go to Add New Theme and find the active theme's card.
3. It should show Activated (disabled) and Customize (linking to the Site Editor) — unchanged behaviour.

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Trac ticket: https://core.trac.wordpress.org/ticket/64729

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Use of AI Tools

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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