Message content "black boxes" in mail threads
In Apple Mail, High Sierra 10.13.1 (17B48), Mid-2011 Mini. Individual messages appear properly, but the entire message - header, quick buttons, and content - is a black box when viewing multiple messages in a thread. Blind double-clicking the header will open the message, which displays properly in a new window. If there's only one message in a thread, it displays in the preview pane correctly as well. This only affects multiple messages in a thread.
Screenshot attached, for what it's worth... there's not much to see there.