Gmail introduced a completely revamped Compose window today. Now, new messages can be written in an overlay on top of your inbox instead of in a separate window.
The new interface is basically the same as Google Chat: when you click Compose, you’ll see a small popup in the bottom corner of your Inbox. From there, you can enter your recipients and write your message while accessing the rest of your inbox.
Now you can reference other emails while writing your current one. It’s a bit simpler than the old Compose screen, but you can still access formatting features and other tools at the bottom of the overlay.
The feature is currently in preview, but you can access it by composing a new message and clicking the “Try It Now” now button that pops up. If you don’t see the option, check back later -- the feature should roll out over the course of the day. Hit the link to read more.
Introducing the New Compose in Gmail [Official Gmail Blog]