Google is introducing a new way to compose messages in Gmail. Instead of loading a whole new "compose" page, you can now start a new email from wherever you are in a popup window. The window isn't actually a new browser window, but rather an overlay on the Gmail page. The new compose looks like a Gmail chat window, though the compose version is a bit bigger.
The new compose window is rolling out as a preview today. If you'd like to try it out just click the compose button and Gmail will ask if you'd like to try the "new compose experience." Hit that link and you'll get the new window.
The change makes it easier to reference old emails while you're composing a new one, since you don't need to leave whatever page you're viewing just to write a new message.
The overlay window also means you can search or keep an eye on incoming mail while you write your new message. Having compose in its own window also means you can write multiple messages at once and even minimize a message to finish it later.
The window within a window model mimics what you'd find on traditional desktop mail apps, and Gmail has also borrowed another feature of desktop mail applications – drag and drop "address chips." The drag and drop features are nice, but the "chips" mean that your recipients' email addresses are collapsed to just the name, which can be annoying if you need to double-check that you're emailing, for example, a person's work address instead of a home address.
For now the new compose window is optional, but Google says that it will, after some "finishing touches," be rolling out to everyone "over the coming months."
[Update: If you really like the new compose window, Google Operating System's Alex Chitu points out that, with a little URL hacking, you can open any message in a compose-style window. Check out Google Operating System for full details.]
