Easy to reproduce:
1. Find a topic.
2. Write some stuff in quick reply
3. Preview -> takes you to full posting page
4. Observe textbox is now sadly empty :(
Thanks for reporting. Can reproduce it on this site, but not on my local copy. Seems someone fixed it meanwhile...
/me whistles innocently trying to fix the bugs he introduced
/me writes stuffs and stuffs.
/me previews, from quick reply, from full post too just because.
Yup, fixed. Thanks!
/me doesn't think it's his fault (or better merit :P)