patch 8.0.0041
Problem:    When using Insert mode completion but not actually inserting
            anything an undo item is still created. (Tommy Allen)
Solution:   Do not call stop_arrow() when not inserting anything.
diff --git a/src/edit.c b/src/edit.c
index d763b53..53b43c4 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -2799,9 +2799,6 @@
 	ins_compl_prep(' ');
     ins_compl_clear();
 
-    if (stop_arrow() == FAIL)
-	return;
-
     compl_direction = FORWARD;
     if (startcol > curwin->w_cursor.col)
 	startcol = curwin->w_cursor.col;
@@ -3876,7 +3873,8 @@
 		/* put the cursor on the last char, for 'tw' formatting */
 		if (prev_col > 0)
 		    dec_cursor();
-		if (stop_arrow() == OK)
+		/* only format when something was inserted */
+		if (!arrow_used && !ins_need_undo)
 		    insertchar(NUL, 0, -1);
 		if (prev_col > 0
 			     && ml_get_curline()[curwin->w_cursor.col] != NUL)