Summary:
It's possible to accidentally collapse "Mailboxes" in the Mail.app source list with no disclosure triangle shown to expand them.

Steps to reproduce:
1) Open Mail
2) Drag another category such as "On My Mac" above "Mailboxes"
3) Collapse "Mailboxes"
4) Move "Mailboxes" back to the top of the source list

Expected results:
"Mailbox" shows a disclosure triangle and so is expandable.

Actual results:
"Mailbox" does not show a disclosure triangle and so is not expandable.