TB 68 offered simple and intuitive up/down navigation between address fields with cursor up/down keys - we should restore that.

STR

1. Composition with pillified To, CC, and BCC recipients (no plain text in address inputs).
2. Use keyboard to navigate from To downwards to BCC, count keypresses.
3. Use keyboard to navigate back from BCC upwards to To.

Actual Result

• extra tab stops on X buttons (Remove the ... field) in both directions
• Shift+Tab is a bit clumsy
• most intuitive vertical navigation with cursor up/down is not available
• regression over TB 68

Expected Result

• allow direct up/down navigation between input fields with cursor up/down, as in TB 68
  • bypass unnecessary tab stops, less clumsy
  • most intuitive
  • maximally efficient

Easy gain.

• For address rows with empty row input, restore easy vertical navigation between input fields via cursor up/down (as in TB 68), as a more agile alternative to Tab/Shift+Tab, which is a bit clumsy and has more stops.
• Ensure that keyboard shortcuts don't fire with unrelated modifiers.
• Rewrite some handlers with early returns and improve comments.

Note: I removed event.stopPropagation(); from Mac-only handling for Cmd+Enter because I don't think it's needed - at least not for the reason given in the comment. Pls test.

Comment on attachment 9207274 [details] [diff] [review]
1696773_composeVerticalCursorNavigation.diff

(In reply to Alessandro Castellani (:aleca) from comment #2)

This is a good improvement for the various addressing fields, but introduces a regression in the Identity field: Pressing Arrow Up or Down changes the identity from the list before moving the cursor to another row.

No, this patch does not introduce that behaviour. That's pre-existing Bug 1635124 which I filed 10 months back.
It would be way out of scope trying to fix that here, and perhaps not even comm-central.
We're not making things worse here than before; there's nothing in the UI promising or indicating that cursor down will work to navigate away from identity selector, and I doubt many users will try that.

Kindly asking the reviewer to reconsider his verdict in view of that.

Ah, sorry about that, I forgot about Bug 1635124.

I think my mistake shows the potential danger of landing this feature before that bug is fixed.
Enabling arrow up and down navigation will make the user comfortable in switching between fields with those keys, making the issue of Bug 1635124 more prominent.

We should find a solution for that bug before working on this.