Disable "Mark as Read" for the Preview Pane only
This has been a frustration for years—I can find posts back to 2012—but it's never been addressed.
I like using the Preview Pane so that I can quickly glance at an email to see what it's about, and whether I need to respond to it right away. But I don't consider the Preview Pane to be an official "read", and I don't want previewed emails to be marked as read—not after 10 seconds, not ever.
Conversely, when I open an email in its own tab/window, I DO consider that an official "read", and I DO want it marked as read—immediately.
But this has never been possible in Thunderbird, and the essential problem is this: the Preview Pane and an opened email are treated as the same action. There is no way to distinguish between them.
Wšě wotmołwy (1)
The 'Preview Pane' is correctly named Message Pane, and viewing a message there is the same as viewing, i.e. opening, the message in a tab or window. The only part of a message that can be considered a 'preview' is the header shown in the Threads Pane with Subject, From, Date etc.