1) Perhaps it's still processing the inputted changes? I see it sometimes and due to host processing power, there is a delay before the changes reflect the preview. And I usually see it on a shared hosting environment, perhaps you can contact your host to increase your allocated memory or do it yourself by adding this code in your wp-config.php
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
2) The problem with the button is, it depends on the height of the content so the only solution would be using flex as described in that post. Perhaps, you could add static width to your button, like a CSS to define its width. Like
margin: auto auto 0 auto;
That's just a sample CSS using the class name provided in that article.