Google is working on this major Gmail quick reply interface change

Google is testing a new quick reply interface in Gmail, which would make it easier for users to respond to emails without having to open a new window. The new interface, which was first spotted by Android Police, is currently available as a limited server-side test from Google and is not yet available to the general public.

The new quick reply interface appears as a small overlay at the bottom of the emails replacing the big buttons for Reply, Reply All, and Forward. It looks like a chat box through which you can reply to new emails.