Mailboxes appear empty, have to click away, click back

Out of the blue (about 2 weeks into 1.0.1, so not update-related), I've started having an intermittent problem in which I go to a mailbox and it appears empty despite having message in it. The only way around this is to click to another mailbox, then click back. When I do, the mailbox pane is full again.

Example screenshot below, showing in the sidebar that there should clearly be at least one (new) message in this mailbox. The image shows a Smart Mailbox, but this happens in my in box as often as anywhere else. It's happened half a dozen times in the last 10 days or so.