Wordpress - Forcing reload of editor-style.css

There is a hook for that: 'mce_css'. It is called in _WP_Editors::editor_settings() and you get all loaded stylesheets comma separated as the first and only parameter.

Now it is easy: Use the global variable $editor_styles (here are your theme’s and parent theme’s editor stylesheets stored already), add the time of the file’s last modification as a parameter and rebuild the string.

As a plugin:

<?php # -*- coding: utf-8 -*-
/**
 * Plugin Name: Refresh Editor Stylesheets
 * Description: Enforces fresh editor stylesheets per version parameter.
 * Version:     2012.07.21
 * Author:      Fuxia
 * Plugin URI:  http://wordpress.stackexchange.com/q/33318/73
 * Author URI:  https://fuxia.me
 * License:     MIT
 * License URI: http://www.opensource.org/licenses/mit-license.php
 */

    add_filter( 'mce_css', 't5_fresh_editor_style' );

    /**
     * Adds a parameter of the last modified time to all editor stylesheets.
     *
     * @wp-hook mce_css
     * @param  string $css Comma separated stylesheet URIs
     * @return string
     */
    function t5_fresh_editor_style( $css )
    {
        global $editor_styles;

        if ( empty ( $css ) or empty ( $editor_styles ) )
        {
            return $css;
        }

        // Modified copy of _WP_Editors::editor_settings()
        $mce_css   = array();
        $style_uri = get_stylesheet_directory_uri();
        $style_dir = get_stylesheet_directory();

        if ( is_child_theme() )
        {
            $template_uri = get_template_directory_uri();
            $template_dir = get_template_directory();

            foreach ( $editor_styles as $key => $file )
            {
                if ( $file && file_exists( "$template_dir/$file" ) )
                {
                    $mce_css[] = add_query_arg(
                        'version',
                        filemtime( "$template_dir/$file" ),
                        "$template_uri/$file"
                    );
                }
            }
        }

        foreach ( $editor_styles as $file )
        {
            if ( $file && file_exists( "$style_dir/$file" ) )
            {
                $mce_css[] = add_query_arg(
                    'version',
                    filemtime( "$style_dir/$file" ),
                    "$style_uri/$file"
                );
            }
        }

        return implode( ',', $mce_css );
    }

I couldn't get toscho's answer to work for the current version of WordPress (4.7.2), and that seems to be because the TinyMCE init array has a cache_suffix set to 'wp-mce-' . $tinymce_version.

So instead, you can just overwrite that with the tiny_mce_before_init filter, like so:

function wpse33318_tiny_mce_before_init( $mce_init ) {

    $mce_init['cache_suffix'] = 'v=123';

    return $mce_init;    
}
add_filter( 'tiny_mce_before_init', 'wpse33318_tiny_mce_before_init' );

Of course, this isn't nearly as good as filemtime(), but at least this works in 4.7.2.

Note: This also adds the cache buster to other editor styles (like skin.min.css, content.min.css, dashicons.min.css, and wp-content.css)

Instead of just calling add_editor_style with your CSS file, add a cache buster query string parameter:

add_action('admin_enqueue_scripts', function(){
  if(is_admin()){
    add_editor_style(get_template_directory_uri().'/assets/css/editor.css?1');
  }
});