Mail hides a draft being edited, triggered by mysterious keyboard event

Quoting my wife:

I'm writing a long article as a draft email. Every week or two I have a panic because my draft disappears! Today's disaster went something like this: I’m in MAIL. I have a long draft email. I bring it up on my screen (but accidentally without enabling editing). I start making a change on it, selecting a few words to change. The changes I want to make are highlighted.  When I discover it can't be edited, I click on “Send” under “Mailbox” and then only the highlighted portion shows on screen. I don’t want that, so I delete that. It goes away and the entire draft is also gone.

Sometimes I can find the draft somewhere, but it's not consistent. This has happened from time to time for years. It's always when I type something, but I'm not sure what.

Today I seem to have lost everything.

Where would it likely have gone?