Guys, please keep this discussion on topic. While I appreciate your honest feedback, this is not the correct place to share it, as Flarum devs won’t see it. If you believe that a certain feature should be implemented in core, please suggest it at https://discuss.flarum.org. Since Flarum is supposed to be a minimalistic and extensible software, I understand why such minor features might be left to be added by extensions. For example, there is the “Rich Text” extension that not only avoids the use of Markdown altogether for better composer experience, but also has support for inserting a horizontal line.
Back to the original matter (medelinor), it appears that you are using the Rich Text extension, which expects three dashes (---) instead of three underscores (___) as the horizontal line. Three dashes is also the Markdown convention for horizontal line, while underscores are used for italic text or bold text - so, I believe what happens is that the three underscores disappear because they are not valid markdown, or because it simply wraps an empty string in bold when you edit the post and the rich editor loads its contents from the markdown, as the concatenated underscores cancel each other out.
In conclusion, please use --- instead of ___ for horizontal lines (or click on the “+” button in the editor controls and on the horizontal line icon). If the issue persists, please contact the author of the extension (https://discuss.flarum.org/d/26455)
