Today we rolled out support for a longstanding feature request we've had here at FastMail: a reading pane in our web interface. Displaying the mailbox listing next to the selected conversation means you can go through your email without switching between two different screens, and you can see at a glance what other messages are in your mailbox whilst reading an email. This works particularly well in today's age of widescreen computers and tablets, making good use of all that horizontal screen space.
You'll find the option to choose a layout that shows the reading pane in the Settings, as part of the "Theme" group of settings. You'll also find here an option to hide the sidebar, which is useful on smaller devices where you want to use the space for the reading pane instead. Note, when logging in on an iPad we automatically enable the reading pane and hide the sidebar to make optimal use of the space available.
The reading pane is not available in the classic interface.