Why is TinyMCE 4.x not using updated CSS even with forced reload? [closed]

I found it. My god, is this poorly documented 😀

TinyMCE not only applies your given stylesheet, it caches an additional copy of it. Because reasons. So if you change something like I did and then control reload everything, you reload the copied cached version. The solution is to add another parameter that you change every time you actually change something in the css. That parameter is called cache_suffix:

$init_array['cache_suffix'] = 'v=SomeVersionNumber';

That parameter is nowhere mentioned in the content_css section – because why should it? – so if you stumble upon this having the same issue, that is how you do it.