How can I ensure every em-dash doesn’t wind up in the following line as the leading character? Even though I don’t include spaces on either side of the dash, Scrivener will still line break between word and punctuation, forcing the em-dash as the leading character in the next line. I’ve searched for this solution in the forums UNSUCCESSFULLY,
Gary
You could use a word joiner (placed in front of your dash, no space), but I’d say the best is to see how it behaves in your destination format/app first. ( ← I mean the dash on its own, no word joiner or anything. … It will probably be fine without any intervention on your part.)
Insert / Break / Word Joiner
I place a space on either side of the em-dash, like so:
—. That prevents the problem you mention AND it prevents situations where two long words, with a dash between, causes tons of whitespace when the line is justified. If there’s an em-space at the end of a piece of dialog, I render it as — ”, where the space between em-dash and closing quote is a non-breaking space. I use a Keyboard shortcut for that, but TextExpander (or Windows equivalent) is also fine for that.
Thanks. But aren’t those two spaces “breakable”? In other words, even though I have hyphenation turned off, I thought all spaces in Scrivener are soft: the em-dash could still end up as the leading character on the next line. Am I wrong? That’s the way it is in most Mac applications, including InDesign. Only in Scrivener have I found the em-dash capable of leading a line.
Gary
That’s why I said you should see in your destination format or final app, and likely don’t have to worry about it.
That is very unlikely, since Scrivener’s text engine is the same stock Mac text engine most native Mac software will also be using. What that could mean is that most of the software you use is not technically native Mac software, or at the least is replacing the text engine (at great expense & effort—which will include some of Apple’s stuff, like Pages, because they don’t eat their own dog food).
To wit, InDesign could barely be called that. Adobe uses their own development environment which is why their software looks so different from everything else. But even if the UI was the same, it would be a massive blunder for a multibillion dollar company making a typesetting and design program to use the free stock Mac text engine, like mere mortals such as we do. 
I must say that @AmberV and the team at L&L certainly know the secrets for seasoning that dog food in a unique way. And I note there have many celebrated dogs that have partken of the food provided here and done very well. I am still looking for the magic (in my writing)-perhaps something for a future show.