Why are there 2 sliders for the box at your forum? Are you using an extension that changes how codeblocks display? It doesn't seem like default Flarum codeblock (but you could be using custom CSS, of course, so I'm asking if it's a bug in the extension you're using or if it's a Flarum bug).
I think that I observed similar bug sometimes before
beta.16/v0.1.0, so it might be a bug in
beta.15 and below too, although I am not sure