Currently, Geary will automatically mark messages in the conversation as read if more than some percentage (30%, IIRC) of the body is visible. There is also a pref to determine if the first conversation in a folder is selected and displayed when the folder is opened. This works well when scrolling down through a long conversation, but results in a kind of data loss (i.e. unread state) if e.g. the autoselect pref is enabled (which it is by default) and the first conversation in a folder is short enough, or when scanning through a number of such conversations in the list. This is in particular a problem for people with vision impairment.

Addressing this will probably call for an automark preference to determine if messages should be automatically marked as read or not, and if enabled should mark individual messages as read only when enough of the message body has become visible, and after a not-too-short delay.

The exactly length of the delay could be a hidden pref, but it would be better to just choose a decent default. It should be a bit longer than a few seconds since it should give people enough time to determine if they are going to actually let it be marked as read or not - maybe 5 or 10 seconds.

It would be good to give some feedback about how long people have left before it is marked as well. Currently the per-message unread indicator line at the top of the message fades out, it would be good if that transition finishes at the same time as when the timer is up and the message is marked.