Using Gmail can be a great experience, but sometimes it’s just nicer to read email in a desktop or tablet app. Having those two panels, one for your list of emails, and one for the actually email content can be much more intuitive. Well, not you can have the same sort of set up int he Gmail web app.
The feature comes in the form of the Gmail Preview Pane option in Labs. Preview Pane lets you split up your screen either vertically or horizontally, depending on how you prefer to browse your mail. Those settings can be switched on and off and will if you enable Preview Pane. The setup seems to work fairly well, though it can be a bit buggy, which is why it is currently in the Labs section of options. Gmail adds a three second delay before marking email read if you use the Preview Pane, which makes sense since it should take at least that long to actually read any single email.
I’ve tried using the Preview Pane and I can say that it’s a nice feature, but I’ll probably disable sometime in the near future. On a 13-inch screen it simply makes the page far too crowded. It looks nicer on a large screen, but if, like me, you use a smaller-screen laptop as your main computer, you might not find it as useful as an app that does the same thing.
Read [Official Gmail Blog]