How to fix markdown-mode's fill-paragraph?

Question

markdown-mode is pretty good at filling paragraphs overall, but there's one flaw that really bothers me. If a paragraph contains a line which ends in two spaces, it only starts filling never fills that line.

For instance, take the following example, where the second line ends in two spaces.

If I hit M-q right here at the start, this line will get broken into two, but the others won't.
If I hit M-q right here only the NEXT line will get split. All because this line here ends in two spaces.__
If I hit M-q right here, this line will get broken into two, but the others won't.

The example explains itself. Even If I hit M-q at the start of each line, I'm still left with this:

If I hit M-q right here at the start this line will get broken into
two, but the others won't.
If I hit M-q right here only the NEXT line will get split. All because this line here ends in two spaces.__
If I hit M-q right here, this line will get broken into two, but the
others won't.

What I would like to have is this:

If I hit M-q right here at the start this line will get broken into
two, but the others won't. If I hit M-q right here only the NEXT line
will get split. All because this line here ends in two spaces.__
If I hit M-q right here, this line will get broken into two, but the
others won't.

Q: How can I get fill-paragraph to behave like this?

Clearly, markdown-mode is considering the entire middle line to be a paragraph separator, when it should be just the spaces at the end.

Answer 1

So one inelegant hammer of a solution for this is to set paragraph-separate as a file-local variable. The mode sets it to "\\(?:[ \t\f]\\|.* \\)*$" as a mode-local-variable; if I set it back to its default value (taken from the help string for paragraph-separate) like so:

-*- mode: markdown; paragraph-separate: "[  ^L]*$" -*-

then I get the behaviour you're searching for:

If I hit M-q right here at the start, this line will get broken into
two, but the others won't.  If I hit M-q right here only the NEXT line
will get split. All because this line here ends in two spaces.__If I
hit M-q right here, this line will get broken into two, but the others
won't.

The comment in the git repo that introduces this says:

Respect hard line breaks when filling paragraphs

A downside of this patch is that paragraph movement commands will also stop at hard line breaks, thus slightly changing the definition of "paragraph" in this mode.

After a lot of tracing, I think I've got the function actually invoked by M-q down to fill-individual-paragraphs; but it's complicated, and even after looking at that comment I still haven't figured out exactly what's going on...thus, blunt hammer, and I don't know what other consequences it'll have. But it seems to work. :-)