diff -urN emacs-26.3-orig/lisp/simple.el emacs-26.3/lisp/simple.el
--- emacs-26.3-orig/lisp/simple.el	2019-08-21 05:16:43 +0900
+++ emacs-26.3/lisp/simple.el	2019-12-22 19:49:20 +0900
@@ -2950,8 +2950,8 @@
 
 (defun undo-auto--last-boundary-amalgamating-number ()
   "Return the number of amalgamating last commands or nil.
-Amalgamating commands are, by default, either
-`self-insert-command' and `delete-char', but can be any command
+Amalgamating commands are, by default, only
+`self-insert-command', but can be any command
 that calls `undo-auto-amalgamate'."
   (car-safe undo-auto--last-boundary-cause))
 
@@ -3016,8 +3016,8 @@
   "Amalgamate undo if necessary.
 This function can be called before an amalgamating command.  It
 removes the previous `undo-boundary' if a series of such calls
-have been made.  By default `self-insert-command' and
-`delete-char' are the only amalgamating commands, although this
+have been made.  By default `self-insert-command'
+is the only amalgamating command, although this
 function could be called by any command wishing to have this
 behavior."
   (let ((last-amalgamating-count
diff -urN emacs-26.3-orig/src/cmds.c emacs-26.3/src/cmds.c
--- emacs-26.3-orig/src/cmds.c	2019-01-07 23:26:07 +0900
+++ emacs-26.3/src/cmds.c	2019-12-22 19:48:15 +0900
@@ -232,9 +232,6 @@
 
   CHECK_NUMBER (n);
 
-  if (eabs (XINT (n)) < 2)
-    call0 (Qundo_auto_amalgamate);
-
   pos = PT + XINT (n);
   if (NILP (killflag))
     {