First time it happened I thought it was a fluke, but then I did it again after spending nearly 10 minutes typing up a detailed comment. If you drag the post comment dialogue down, it takes you back to the thread. This loses what you’ve typed and has no confirmation, nor is the text still there when you reopen the comment dialogue.

I’ve managed to do this twice entirely by accident (once when I was trying to scroll down to reread the comment I was replying to).