View > Layout > Wide/Classic/Vertical
If you reduce width of Message Pane to the point where the Message Pane buttons 'Reply, Forward, Archive,Junk, Delete, More' start to occupy full width, then as you further reduce the width, the vertical scrollbar will disappear completely and Buttons and Header and text/Body content of message becomes truncated.
If there is also a horizontal scrollbar, this will function in part to reveal some hidden text, but the right end of the horizontal scrollbar is also missing. Using the horizontal scrollbar does not reveal the vertical scrollbar.
So if all the Message Header buttons display then there is no issue, but if you reduce width so those buttons cannot be fully seen then the problem occurs.