Ah, I have something to write about. That means I can put off the whole Big Post Of Doom until tomorrow, when I have time to write it.
People with whom I've been corresponding by email over the last few years will have noticed something rather peculiar: my emails are often squashed over to the left-hand side of the screen, with a line length of about 80 characters. Now, that's just fine: the email standard specifies a maximum legal line length of 76 characters (back in the day when you did everything at the terminal, that's all that would fit on screen, and no-one's seen the need to change it). However, in this era of wide-screen monitors it does look a little ... odd.
It turns out that GMail enforces the 76-char limit in its plain-text mode, by forcing breaks where they're not present. I do most of my email in plain-text mode, since I have little use for the extra features HTML email brings, but I hadn't noticed that there was any other editing option in GMail. On the top-left corner of the "Compose Message" box, there's a link that says either "Rich formatting" or "Plain text": whichever text you see, it's in the opposite mode. I've just flipped my account from plain-text to Rich Formatting mode, and the spurious line breaks have gone away (since it can do what it likes to the line-length of the HTML source, but the changes won't be displayed at the opposite end). This may or may not be a long-term fix, but it appears to have positive side effects.
If you're one of the N people who have responded "WTF?" to a slightly oddly-formatted email in the past, hopefully the problem has gone away. Enjoy :)