There's another requirement in this project: a responsive design that works on devices with smaller screens. Well, it turns out that the UI we've come up with already looks pretty good on a tablet, with one caveat. You have to be holding the device in landscape mode. This is a problem that you don't really have to consider on the desktop, but it becomes critical with mobile devices.

From an architectural standpoint, we need to understand how the layout of the application will differ between screen sizes in order to decide how to assemble the application. In the designs so far, there are two panes side by side; there simply won't be enough screen space to allow for this on a portrait phone or tablet.

Instead, we'll hide or show the "left" and "right" panes depending on user actions. If they click on a thread of messages or the New Message button, the threads will be hidden and the correct right-hand pane will appear.

The only other issue with a portrait screen is...