I have a win32 03/25/01 Trunk debug build that I am using to read my IMAP mail. I had about 500 new messages, and when I got a quarter of the way through reading/deleting these new messages, the Mail Headers stopped rendering. That is I would click on a message, and only the body of the message would display, at the top of the message pane content window as if the message header toolbars were hidden.
*** This bug has been marked as a duplicate of 39655 ***
Marking verified as a duplicate. Kin: Drag the thread/msg pane splitter when this happens to resize the thread/message panes and the headers will display.