To reproduce:
- narrow the main window so the message list has a horizontal scroll bar with about 1/2" (1cm) available motion
- slide the bar
- note how the column headers' icons/text are all blanked out a pixel at a time as the scroll bar is moved

If the main window is narrowed further, with say 2" (5cm) of scroll space; the effects are more destructive of the headers (not permanent). 

Probably should be a separate bug report; but the code handling the headers is quite brittle:
Changing the width of the {lock,mark,status,attachment} column adds more confusion to the display requiring manual editing of clawsrc and a CM restart to clear up.  Maybe the icon columns should be a fixed size based on content; it's not intuitive to get them back to reasonable sizes without manual edits and restarts.