gmail broken with arbitrary, invisible, forced short autowrap

 ... or that's how it looked to me. 

Gmail compose took a long input line, which normally would autowrap within whatever box it was viewed, but instead created a hard, short autowrap, in the background, where I could not see it, and did not want it.

Obviously I'd accidental changed a setting. But I scoured the settings, and couldn't see an option that fit the problem. 

There's another set of settings, weirdly not referenced or linked in the main settings. These are in the compose window. And they don't apply until the next time you open the compose window.

Those settings are under the three dots to the right of the text tools, and the culprit was "plain text mode".

Now as someone who used email decades before there was email formatting, I was a little irked by the assertion implied by this setting's name.

If it was 'plain text' why not just treat the input the way it will be received? Why create an arbitrarily short autowrap of the input text, which will alway look wrong? This is because it's not previewed, that is, it's not WYSIWYG. It turns a potentially useful option into one that would only be useful for sending emails to very primitive small-screen devices, with no option to use plain text in a way that's under control of the sender. 

So, a broken UX in gmail. Which usually is more careful about its features.