To enable the setting, go into Gmail Settings, then the Labs tab, and finally enable "Preview Pane." Once you do that, you'll see a toggle button in the top right corner of your message list. It will allow you to switch between vertical / horizontal preview and list views (dubbed vertical split, horizontal split, no split).
Vertical split means the preview pane is on the right of the inbox; horizontal split means the preview pane is beneath the inbox.
[Unfortunately, the vertical preview doesn't seem to work in Opera 11.5 build 1074. Sigh.]
In addition, the feature also allows you to adjust how many seconds a user can spend looking at a message's preview before that message is marked as read; the default setting is three seconds.
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