In Mail & News, nightly build ID: 2002101708, the arrow to expand folder lists for IMAP accounts has dissapeared. Using "Modern" theme, presently. Clicking the area where the arrow should be allows me to expand the folders, and the arrow then appears facing down, but dissapears again when the list is collapsed. See the listed URL for an illustration of what I mean. The red oval contains the area where arrows should be.
*** This bug has been marked as a duplicate of 174831 ***