I don't remember that ever happening to me before and when I tried it just now, I couldn't get it to happen. If I place the "cursor" somewhere where I can't actually insert text, it seems to make no difference if I then try to type or not. I can see how, if I had my real text (farther down) highlighted, it could very well disappear at the first false move.

I suspect that your browser is at least part of the problem. My previous browser, the one I abandoned 6 years ago, was always crashing or eating posts on me. My current browser, which is no longer being supported but still works for me, is amazing that way. The "Undo" feature really does reverse the last thing I did, whether typing a phrase or inserting a pair of tags.

One time I was far along in a long post when the power went out in my neighborhood. Naturally I expected to have to start the post over from scratch but when I powered up the computer again an hour or two later, my browser somehow managed to take me to the same page with most, maybe even all, of my post still in place. I was impressed!

Here's something you might want to try out ahead of time when you're working on a post that's easily replaceable (or that you have a copy of). When you get to the place where you could make it disappear if you did something -- do that on purpose, make the post disappear, then see what happens when you right-click and choose "Undo".