Using firefox 3.6.17.
Tried with Midori (GIT master), some Chromium version from Debian Testing and Firefox 4.0. I couldn't reproduce it but that doesn't mean much.
Not happening for me at the moment either, maybe its some thing to do with exactly what is in the content, maybe something strange I did, although I thought it happened on an unmodified playground, but maybe I had changed something.
The disabled save button comes from the little extra javascript I installed yesterday. I could remove it to check whether this is related? However, I can't imagine this javascript breaks the whole layout at preview.
Sorry to not be clear, the save button is not greyed out, but the the sidebar actually overwrites the left hand strip of the edit window and the save button. Attempting to click save selects the items in the sidebar menu not the save button. So I had to save before previewing or lose the changes. Thats probably not your javascript which is worthwhile to remind us to fill in the comment :-)
BTW I put the images in a howto:images: namespace rather than directly in howto.
Cheers Lex