diff --git a/src/keyboard.c b/src/keyboard.c
index 6895a249f22..a99d14cb4cf 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -3462,6 +3462,8 @@ readable_events (int flags)
      READABLE_EVENTS_FILTER_EVENTS is set, report it as empty.  */
   if (kbd_fetch_ptr != kbd_store_ptr)
     {
+      /* See https://lists.gnu.org/r/emacs-devel/2005-05/msg00297.html
+	 for why we treat toolkit scroll-bar events specially here.  */
       if (flags & (READABLE_EVENTS_FILTER_EVENTS
 #ifdef USE_TOOLKIT_SCROLL_BARS
 		   | READABLE_EVENTS_IGNORE_SQUEEZABLES