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How to fix Gutenberg with SiteOrigin page builder deleting content when editing

WP 5 how to add_editor_style for both tinyMCE and Gutenberg

You need to call add_theme_support( ‘editor-styles’ ) in the after_setup_theme hook. More information

WordPress Gutenberg home page “edit Page” option missing

The posts page, which is always index.php (main template) of your theme, cannot be edited by neither, Guttenberg nor Classic Editor. Create a child theme, and copy index.php from your parent theme to the child theme. Then you can edit your child’s index.php, using external text editor.