So, I’ve been meaning for a very, very long time (over a year) to post something I discovered related to a question from another user on the old forum a couple-three-plus years back. I can’t find that thread in my logs, so I hope the party(s) looking for a solution to a maddening issue for many years catch this.
In macOS Mail.app, if you use a 3-pane layout, when you have your focus on the list of messages, the selected message, is, of course, blue (or whatever you have chosen in System Preferences –> General –> Highlight color); but if you click your cursor elsewhere, e.g., a link or image in the selected email in the preview pane, the selected list item goes gray (expected), and keyboard navigation is now broken; you can no longer use Up/Down Arrow or Command-[left square-bracket or right square-bracket] to navigate the list; and no amount of Tab key presses will ever, ever return you to that column, including Shift-Tab, Option-Tab, etc…
For years this (seemingly) forced the user to return the cursor to the list column and select/re-select an item to continue keyboard navigation.
So I don’t know when or how this broke, but I can confirm it’s broken in El Capitan thru at least Sierra (I forget to test it every time I have an older VM open).
But ‘broken’ is apparently subjective, because it Turns Out!™ that there’s a totally unintuitive (to me) way to return focus.
Assuming you have the Favorites Bar visible, and you have populated it with up to nine (actionable) folders or smart folders, just like in Safari, you can use Command-[1-9] to switch focus to a given folder, numbering from left to right. Awesome sauce to begin with.
So, one day, moons and moons ago, I’d lost focus, but I wanted to go to another mailbox view, anyway, so I went to hit the corresponding keys, and accidentally hit the keys for the current mailbox. BOOM! focus returned. Purely a discovery-only item; not documented anywhere I can find; and the issue totally breaks with UI conventions in the first place – it should be accessible with Tab, just like every other object in the main browser window.
I’m sure a bunch of people smarter than me have probably known about this forever, but since I was Entourage and Outlook for a (too) long time, I never really got super-proficient in Mail.app.
Anyway, in the above example, ‘Inbox’ is first, so Command-1 will bring it into focus. If you keep finding yourself losing item focus, just make sure your favorite folders are in the Favorites Bar, learn to use the numbered shortcut combos, and never reach for the mouse again to keep your sanity when navigating.