Custom Format div in TinyMCE Merges with previous div

Add these functions to your theme's 'functions.php' for registering new button in TinyMCE.

// init scriptsadd_action( 'after_setup_theme', 'custom_tinymce_theme_setup' );if ( ! function_exists( 'custom_tinymce_theme_setup' ) ) {    function custom_tinymce_theme_setup(){        add_action( 'admin_init', 'custom_tinymce_theme_add_editor_styles' );        add_action( 'init', 'custom_tinymce_buttons' );    }}// add editor custom cssif ( ! function_exists( 'custom_tinymce_theme_add_editor_styles' ) ) {    function custom_tinymce_theme_add_editor_styles() {        add_editor_style( 'custom-editor-style.css' );    }}// add editor button filterif ( ! function_exists( 'custom_tinymce_buttons' ) ) {    function custom_tinymce_buttons() {        if ( ! current_user_can( 'edit_posts' ) && ! current_user_can( 'edit_pages' ) ) {            return;        }        if ( get_user_option( 'rich_editing' ) !== 'true' ) {            return;        }        add_filter( 'mce_external_plugins', 'custom_tinymce_add_buttons' );        add_filter( 'mce_buttons', 'custom_tinymce_register_buttons' );    }}// call editor custom jsif ( ! function_exists( 'custom_tinymce_add_buttons' ) ) {    function custom_tinymce_add_buttons( $plugin_array ) {        $plugin_array['wrap_tab'] = get_template_directory_uri().'/js/tinymce_buttons.js';        return $plugin_array;    }}// add button to editorif ( ! function_exists( 'custom_tinymce_register_buttons' ) ) {    function custom_tinymce_register_buttons( $buttons ) {        array_push( $buttons, 'wrap_tab' );        return $buttons;    }}

Then create a new JS file named 'tinymce_buttons.js' under your '[theme]/js' directory and add the following code.

(function() {    tinymce.PluginManager.add( 'wrap_tab', function( editor, url ) {        editor.addButton('wrap_tab', {            title: 'Insert Tab',            cmd: 'wrap_tab',            text: 'Tab',        });        editor.addCommand('wrap_tab', function() {            var selected_text = editor.selection.getContent({                'format': 'html'            });            if ( selected_text.length === 0 ) {                alert( 'Please select text that needs to be wrapped.' );                return;            }            var return_content = '<div class="tab">' + selected_text + '</div>';            editor.execCommand('mceReplaceContent', false, return_content);            return;        });    });})();

You can then include the CSS styles - create a new CSS file named 'custom-editor-style.css' under your theme folder with style definitions to your DIV element.

#tinymce .tab{ padding:10px; border: 1px #666 solid; }

Hope this helps and credits goes to https://madebydenis.com/adding-custom-buttons-in-tinymce-editor-in-wordpress/.