It's very rare and I can usually click in the text box then Control A, then Control C to copy the text to Windows clipboard in case it doesn't come back.
PS. Only happened to me a year ago but browsers and such are updated and I haven't seen it since.
Sometimes when I'm proof-reading something I'm about to post, I want to click back into the text, but when I do, the cursor disappears. Neither a mouse click nor the arrow keys will move it. This doesn't happen in any other editor I've ever seen. Is this a known bug or what?