The forum software has an innate dislike for horizontal rules. None of the typical Markdown encodings work
[at which point I started entering codes to demonstrate their not working, until it finally dawned on me the primary indicator of their not working is that you can’t see them. Suffice to say neither — (3 hyphens), *** (3 asterisks), nor ___ (3 underscores) on the left margin will trigger a horizontal rule; likewise, the 10- hyphen block inserted with Ctrl-Alt-R (as suggested above) also disappears.
However, entering even a single hyphen on the line immediately beneath some text does interesting things to the previous text block.
And while I have you here, what’s up with a preformatted (code) text block that doesn’t retain leading white space, eliminating indentation? Pretty much wipes out the reason for quoting code.