I reloaded the page. While the forum post itself never changed - even when restarting firefox or reloading the page with CTRL+F5 - I noticed that the preview for editing the post does use the modified settings when reloading the page.
So, with cpp as only language in the list (to keep things simple), if I set the default language to cpp, code highlighting in the post edit preview is correct. If I set the default language to auto and again reload the page, it’s not correct anymore as described in the previous post.
Kind regards,
Max