A recent update of Chrome browser broke the WordPress admin panel. It happened because of a setting related to slimming paint. Of course you can fix it by going to chrome options and then by disabling it. However, your client may not be able to do it because it’s a pretty confusing setting. To stay in the safe side, it’s nice to add this little snippet in your theme and it will make sure that everything works nicely

[code lang=php]
function admin_menu_chrome_bug_fix() {
echo '<style>
#adminmenu { transform: translateZ(0); }
</style>';
}
add_action('admin_head', 'admin_menu_chrome_bug_fix');
[/code]

Enjoy!