I've had that happen to me lots of times (mostly elsewhere so far though, but a couple of times on here)
Usually I just go "screw it, you know what, I guess they don't need to read it because I'm not typing all that again" and instead condensate my post into just one sentence if I had something I felt was meaningful to say on a topic.
However, I'm sure a browser plugin or something exists that tracks what you've just typed, if you're not worried about the ramifications of possibly leaking the text to whomever created that plugin. But hey, isn't that what we're doing anyway, typing text that we usually hope as many people will see as possible?