It's funny. I reentered what I wrote (it was in Sang Sacre, so it was long, too) edited it, and then was able to go back to the edit page and it was still in the posting box.
What I used to be able to do on WX was when I was mearaing, I could copy some text on a page, paste it in the posting box, go back a couple of pages, grab another piece of text, go forward to the posting box, paste the new text, and the original material I pasted would still be there. It seems that on the Phoenix, the Back and Forward buttons act in the same manner as the Previous and Next links do, redrawing the page every time, rather than just moving from one cached page to another. Except for the edit page.