Geary is currently somewhat inconsistent and incomplete in how it lets people manage the lifecycle and labelling of conversations. This is in part due due to trying to fit the conversation model to both IMAP and GMail, and part due to feature growth over the years. We should tidy this up to make it both consistent and complete.

Based on some notes I made, conversations seem to exist in two different finite state machines — one that covers labelling of conversations, i.e. conversations that should be kept, and one that covers the overall conversation lifecycle. States sometimes represent mailboxes, but sometimes are an abstract state, and the transitions between these states define the actions that people might want to take. These actions should be made available to people as high-level UI items, and Geary should manage the implications of the actions under the hood.

Labelling FSM
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States:

 - Inbox
 - Archive
 - User defined mailboxes

Transitions:

 - Archive
 - Move & Copy
 - Restore 

Lifecycle FSM
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States:

 - [Normal] (Any conversation in the FSM above)
 - Draft
 - Trash
 - Junk
 - [Deleted]

Transitions:

 - Trash
 - Junk
 - Restore
 - Delete

See the attached diagrams for what transition is applicable for each state.

User Experience
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A conversation's current state defines what UI is presented to the user when it is selected. E.g. in the Archive state these actions should be available: Restore, Copy, Move, Trash, Junk.

Archive and Trash are primary action for conversations in the Inbox and so they should probably be emphasised and easily activated via the UI.

The Restore action is the only primary action for most other states, and hence that should be similarly emphasised, although implementing this properly will be tricky, e.g. to ensure that sent mail in the conversation is restored to Sent, and received mail in the conversation is restored to whatever other mailbox(es) it was in.

So I'm imagining that Archive, Trash and Restore should be toolbar buttons with text labels, while the rest can be icons?

Notes
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Still to be defined is how these actions apply to individual messages in the conversation. E.g. Should Trash move just the messages in the current mailbox to the Trash (as at the moment), or all messages (e.g. including any in Sent?)

Comments welcome!