Currently, Geary's composer has 4+ modes, requires you to detach the composer to view a different conversation, and displays a variety of different dialogs in different situations when it needs to be closed. We also want to show a composer by default (Bug 792362), show drafts as inline composers (Bug 730486), and so on.

One way to solve a number of these issues is to keep a single composer open at all times at the bottom of the main window, in the conversation viewer's column. Normally this composer would be minimised, but when activated it would expand to fill half the column when active, or fill the whole column when a new conversation is started, i.e. the "Compose" button is clicked. An active composer could be detached as at the moment in any case.

When viewing an existing conversation, the minimised composer would show a "click to reply" label, which when the user clicks on or if they select Reply to Conversation, would activate the composer and populate it as a reply to the last message, as at the moment. If Reply To is selected for a specific message, the same occurs, but with respect to that message. If Reply To is selected for additional conversations or messages, they are added into the existing composer.

Drafts would not be displayed as inline composers, rather as messages in the conversation with an info bar similar to the one used in the Drafts folder at the moment, and when the user clicks the Edit button, the composer is activated and the draft is loaded into it. If the composer already contains a message when loading a draft, that is automatically saved as a draft if supported, or the user is prompted if they wish to lose the existing composed message instead.

When a different conversation or folder is selected, any activated composer simply remains in place, allowing users to go search for other relevant messages without detaching the composer or being prompted by a dialog. If the composer is full-height, it is shrunk down to be half height but otherwise remains in the window. If the user does navigate away from the current conversation like this, we might want to show a button to allow users to easily return to the conversation being replied to.

Lastly, taking a cue from the GNOME Mail designs, putting the composer's controls in an action bar at the bottom of the window removes weirdness with the GtkHeaderBar changing depending on if there is a composer present or not, allowing users to do a find in the displayed conversation when replying to it.