(M) I have started to reply to several messages and then had to leave that composition window to re-read another message or check spelling only to find my message was no longer there when I returned.
(M) If you are writing more than a one liner, consider Copy-Pasting your message to your regular email or a word processing program before you move to any other window so if you lose it you can always Copy-Paste it back if necessary.
This seems to be a failing of many forums so it may come with the territory.
Ciao,
Marcel