I have a Roots site (https://epoch.exaktime.com) hosted on WP Engine with an SSL certificate. When I posted a comment on a blog, the screen turned white. According to support at WPE:
+1 to @kalenjohnson’s reply. Afaik there are only a few WP functions that don’t run a protocol check. With the exception of wp_get_attachment_url() they’re mostly pretty obscure functions…
Also, this should not be an issue if you set your site URL to use https directly in wp-admin.