1 files changed, 4 insertions, 1 deletions

diff --git a/src/frame.h b/src/frame.h
index 53c0401d43b..f714fc063ce 100644
--- a/src/frame.h
+++ b/src/frame.h
@@ -241,7 +241,10 @@ struct frame
   /* Number of elements in the vector that have meaningful data.  */
   int menu_bar_items_used;
 
-  /* Width of the scroll bar, in pixels and in characters.  */
+  /* Width of the scroll bar, in pixels and in characters.
+     scroll_bar_cols tracks scroll_bar_pixel_width if the latter is positive;
+     a zero value in scroll_bar_pixel_width means to compute the actual width
+     on the fly, using scroll_bar_cols and the current font width.  */
   int scroll_bar_pixel_width;
   int scroll_bar_cols;
 };