Disabling Mark as Read Automatically
This is a feature request as discussed here: https://mail.gnome.org/archives/geary-list/2019-January/msg00002.html
To summarize the request from the discussion:
Preferences should have a new option: "Mark messages read when replying, archiving, or taking other actions"
If the option is NOT selected, conversations are marked as read when they are loaded and reading progresses, wich is the current behavior.
If the option IS selected, then taking any action (replying, forwarding, archiving, trashing or junking) will mark the message that the action is taken on and all previous messages in the conversation as read.