Query Monitor is a comprehensive WordPress debugging plugin that extends far beyond database query monitoring to provide complete theme and plugin debugging capabilities. The admin bar integration provides immediate visual feedback on PHP errors, warnings, and notices.
Key debugging features for theme developers include request analysis (showing matched rewrite rules and responses), template hierarchy visualization (current template plus fallback options), and conditional tag status (is_single, is_page, etc.).
The blocks panel lists all first-level blocks on the current page with attributes, context data, render callbacks, and performance timing. This helps debug block-specific issues and understand page composition.
Additional panels cover hooks in use (showing all callbacks attached to specific actions/filters), enqueued styles and scripts (with dependency information), and complete request debugging data for custom post types and taxonomies.
Important: Only enable Query Monitor on development and staging sites, never production, as it collects extensive data that impacts performance.