diff --git a/lisp/faces.el b/lisp/faces.el
index 84180eadc03..9ec20c42987 100644
--- a/lisp/faces.el
+++ b/lisp/faces.el
@@ -2878,7 +2878,11 @@ Note: Other faces cannot inherit from the cursor face."
      :background "grey96" :foreground "DarkBlue"
      ;; We use negative thickness of the horizontal box border line to
      ;; avoid enlarging the height of the echo-area display, which
-     ;; would then move the mode line a few pixels up.
+     ;; would then move the mode line a few pixels up.  We use
+     ;; negative thickness for the vertical border line to avoid
+     ;; making the characters wider, which then would cause unpleasant
+     ;; horizontal shifts of the cursor during C-n/C-p movement
+     ;; through a line with this face.
      :box (:line-width (-1 . -1) :color "grey80"))
     (((class color) (min-colors 88) (background dark))
      :background "grey19" :foreground "LightBlue"