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After you enable Preview Pane from the Labs tab in Gmail Settings you’ll see a toggle button in the top right corner of your message list, which lets you switch between preview and list views.
For those of you who have more vertical space you can also move the preview pane below your message list. You can enable this using the dropdown arrow next to the toggle button:
By default there is a 3-second delay in marking a conversation as read after previewing it. If that doesn’t feel natural to you, you can change the timing in the General tab of settings:
If you’d like to remove the new layout, simply return to Gmail Settings and disable the lab.