diff --git a/mercurial/help/config.txt b/mercurial/help/config.txt
--- a/mercurial/help/config.txt
+++ b/mercurial/help/config.txt
@@ -1756,6 +1756,9 @@
 possible. Some progress bars only offer indeterminate information, while others
 have a definite end point.
 
+``debug``
+    Whether to print debug info when updating the progress bar. (default: False)
+
 ``delay``
     Number of seconds (float) before showing the progress bar. (default: 3)