Message window spontaneously rewraps
When a text part shown in either the preview or a message window has lines that are long enough to be wrapped, moving the pointer over the 'attachments' button (paper-clip icon) sometimes make them rewrap wider. Since the text no longer fits into the window, the bottom scrollbar is shown and must be used to view all the text.
That button is packed at the end of the header block, which also becomes wider, moving the button to the right; it may become partly or wholly obscured by the right edge of the window.
Like #2 (closed), this issue is related to setting margins, and it is fixed by the same merge request, gtk!236 (merged).