Geary will have a new, and useful feature, in the next release

 member of Yorba team announce that, a new major feature will be 
available on Geary, on the next stable release.

An inline email composer.

The new feature that will come with Geary, it's a embedded composer in the right window bellow the message, when you want to reply an email, instead of a new window popping up on the screen.
Although, if you want a separate window, it only need to press the Detach button and you can reply the emails just like Geary used to work.

Source: Yorba Foundation

This great addition to Geary is only available on git master, and it will be available for general public on the next stable version of Geary 0.7.