Start Emacs under X11 with -q -no-site-file, then type `M-x
set-frame-font RET' and at the prompt type
`-misc-fixed-medium-r-normal--13-120-75-75-c-70-iso8859-1 RET'. Now
type `M-x ruler-mode' twice, first to display and then hide the ruler.
You will now see a vertical line the height of the ruler in the upper
left margin, between the scroll bar and the fringe. Similarly, typing
`M-: (setq mode-line-format nil)' leaves a vertical line the height of
the mode line in the lower left margin, between the scroll bar and the
fringe. Further observations:
- These residues of the mode line and header line (if that's what they
are) disappear when the frame is redisplayed (e.g. by typing `C-l').
- Unsplitting a vertically split frame also leaves such residues.
- I've also observed residues of the header lines in tabbar-mode and
w3m-mode, but the Info header line appears to leave either a very
faint residue or none, on my monitor with the default colors.
- The residues appear only with certain fontsets. I haven't tried all
the fonts on my system, but I've only observed the residues with
fonts with a width field of 70.
- The residues appear only when the scroll bar is on the left.