I don't do too many "tech support" posts here, but I thought I'd add this one because a) it was hard to find the way to fix it, and b) if frustrated the shit out of me.
I recently switched my Yahoo! mail over to the new beta version. I like the look and feel of the new version, but after a day or two of using it, the panel that allows you to preview a message on the bottom half of the screen had disappeared, and there was no apparent way of bringing it back.
I looked around the Yahoo! site for a couple of days, but found nothing that addressed the issue. I finally tried using Yahoo's chat service, where I was told by two agents on two separate chats that the new mail didn't have a preview pane. I'm not sure what made me think they did....Oh, wait a minute, yes I am--they advertised it as a major feature of the new version! Idiots.
So I kept looking and finally found the answer. I hope this will save some others the hassle I went through.
1) Go to the "options" button on your Yahoo! mail page and select "more options."
2) Under "general" options, look for "scroll," and make sure to select "Each panel on the page individually." (by default, Yahoo! has selected the other option--"Content of the entire page," which removes your preview panel. This was a lousy choice on their part, I believe.)
3) Make sure to save your options. Yahoo! will then refresh (so make sure you're not in the middle of writing a message) and voila--your preview panel should be back!
If you'll indulge me for a second, I'm just going to throw a few key words and phrases at the bottom of the post for those people who are doing a search for this issue.
Keywords: Yahoo! mail beta, missing preview button, preview window disappeared, yahoo preview gone.