Multisite: Fix My Sites admin bar menu for large networks.

[soderlind]

Multisite: Fix My Sites admin bar menu for large networks

Trac ticket: #15317 (open since 2010)
Based on: Learnings from the my-sites-fix proof-of-concept plugin and the Super Admin All Sites Menu plugin.

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The problem

The "My Sites" dropdown in the WordPress admin bar has three long-standing issues on multisite networks:

1. Super admins only see sites they are explicitly added to. get_blogs_of_user() discovers sites by scanning user meta keys ending in _capabilities. A super admin has implicit access to all sites but only has explicit capability rows for sites they were individually added to — so the menu is missing most sites.

2. The dropdown can't scroll past the viewport. When the site list is longer than the browser window, items below the fold are simply unreachable. There is no max-height or overflow on the submenu wrapper. This is the original bug reported in #15317.

3. switch_to_blog() is called for every site. Both wp_admin_bar_my_sites_menu() and get_blogs_of_user() call switch_to_blog() per site — the former explicitly in a loop, the latter implicitly via WP_Site::get_details() when accessing $site->blogname and $site->siteurl. On a network with hundreds of sites this is a significant performance hit.

What this patch changes

Files changed

File	Summary
user.php	Super admin branch in get_blogs_of_user(); batch option fetch; new _batch_get_site_options() helper
admin-bar.php	Rewrite per-site loop in wp_admin_bar_my_sites_menu()
admin-bar.css	Scrollable dropdown + fixed-position fly-outs
admin-bar.js	Fly-out positioning with viewport clamping

Fix 1 — Show all network sites for super admins

In get_blogs_of_user(), when is_super_admin( $user_id ) is true, the function now calls get_sites() (ordered by path, excluding archived/spam/deleted/mature) instead of scanning _capabilities meta keys. This ensures the admin bar and the My Sites admin page show every site on the network.

Fix 2 — Make the dropdown scrollable

A new @media screen and (min-width: 783px) block in admin-bar.css adds:

#wpadminbar .ab-top-menu > li#wp-admin-bar-my-sites > .ab-sub-wrapper {
    max-height: calc(100vh - var(--wp-admin--admin-bar--height, 32px));
    overflow-y: auto;
}

This makes the My Sites submenu scrollable when it exceeds the viewport height. The mobile menu (below 783px) is unaffected.

Caveat: overflow-y: auto clips the nested fly-out submenus (Dashboard, New Post, etc.) that normally position themselves with margin-left: 100%. The fix switches those fly-outs to position: fixed and computes their coordinates in JavaScript using getBoundingClientRect(). The JS also clamps the fly-out's top position so it never extends below the viewport, and supports RTL layouts by positioning to the left of the parent item when document.documentElement.dir === 'rtl'.

Fix 3 — Eliminate switch_to_blog() overhead

In the admin bar menu loop: wp_admin_bar_my_sites_menu() no longer calls switch_to_blog() per site. URLs are computed directly from the blog object properties ($blog->siteurl, $blog->domain, $blog->path). Per-site current_user_can() checks are removed — these were guarding convenience links (Dashboard, New Post, Manage Comments) whose target pages enforce their own permissions.

switch_to_blog() is still called when site icons are enabled, since has_site_icon() and get_site_icon_url() require switched context. To mitigate the cost, the wp_admin_bar_show_site_icons filter now defaults to false for networks with more than 20 sites (previously it defaulted to true unconditionally). The filter remains available for plugins to override.

In get_blogs_of_user(): The magic property accesses $site->blogname and $site->siteurl on WP_Site objects triggered WP_Site::get_details(), which internally calls switch_to_blog() for each site. This is replaced by a new private helper function _batch_get_site_options() that builds a single UNION ALL query across per-site wp_N_options tables to fetch blogname and siteurl for all sites at once. Table names are derived from $wpdb->get_blog_prefix() (a pure function with no side effects), and option names are passed through $wpdb->prepare().

Backward compatibility notes

• The pre_get_blogs_of_user filter (since 4.6.0) continues to work and can override the super admin behavior.
• The get_blogs_of_user filter continues to fire on the result.
• The wp_admin_bar_show_site_icons filter (since 6.0.0) still controls site icon visibility; the only change is the default value now considers site count.
• The myblogs_blog_actions filter in my-sites.php is not affected — that page's render loop still uses switch_to_blog() to preserve backward compatibility for plugins relying on switched context.
• The _batch_get_site_options() function is marked @access private and is not intended for external use.

Testing

1. Multisite with < 15 sites: Dropdown should not scroll unless needed; fly-outs appear correctly.
2. Multisite with > 25 sites: All sites reachable via scroll; fly-outs clamp within viewport.
3. Super admin vs regular user: Super admin sees all network sites; regular user sees only their sites.
4. Performance: Verify no switch_to_blog() calls during menu render (except when site icons are enabled for ≤ 20 sites). Check via Query Monitor or $GLOBALS['_wp_switched_stack'].
5. RTL: Fly-out submenus should appear to the left of the parent item.
6. Mobile: Menu unchanged below 783px viewport width.
7. No-JS fallback: Scroll works (CSS-only); fly-out positioning falls back to top: 0; left: 0.

Trac ticket: https://core.trac.wordpress.org/ticket/15317

Use of AI Tools

GitHub Copilot + Opus 4.6 was used to review this fix

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[soderlind]

@westonruter ^^