WordPress 6.3 introduced a major change by placing the content editor inside an iframe, fundamentally altering how CSS styling works. This change affects any theme or plugin that uses the enqueue_block_editor_assets hook to add styles.
Previously, the content editor was part of the main admin page, so any styles added via enqueue_block_editor_assets would affect both the admin interface and the editor content area. The editor was simply integrated into the overall admin page structure.
With the iframe implementation, the content editor becomes completely isolated from the rest of WP Admin. This means styles enqueued on the admin page no longer reach the editor content, breaking many existing styling approaches.
While this change initially seems disruptive, it actually solves longstanding problems with style bleeding between admin and editor contexts, and creates consistency across all WordPress editors.