I think there is a time limit on inactivity. Typing in the message box must be seen as inactivity. lol

p.s. good idea to write the message elsewhere, or at least copy it before hitting "Submit Reply" especially if it's a really long message. My hands have their own minds sometimes and do things my brain is not telling them to do so if I've spent ages writing a long message and suddenly my fingers hit certain keys that I'm not meaning to hit, then POOOOOF.... all gone!