Google introduced a new way to compose messages in Gmail. Instead of loading a whole new “compose” screen, you can now start a new email in a popup window. The new composelooks like a Gmail chat window, though the “compose” version is a bit bigger.
The new compose window is rolled out by Google. If you’d like to try it out just click the compose button and look for the message Try out the “new compose experience” beside Labels. Hit that link and you’ll get the new window after a page loads in seconds. For now, you can switch to the old interface if you don't like the changes.
The change makes it easier to reference old emails, drafts 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 composed in its own window also means you can write multiple messages at once and even minimize a message to finish it later.
Google also improved the interface for replying, but it looks like you'll still reply inline by default. "The reply experience has been designed to fit better inline as part of your conversation - replies take up much less vertical height, intelligently expands to fit your content, and always keep the recipients and other controls in view no matter how long your message gets."
Gmail is suddenly more difficult to use, but the new interface looks better. 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.”