--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -2591,7 +2591,7 @@ A jump table for the options with a shor
 	security reasons.
 
 						*'dip'* *'diffopt'*
-'diffopt' 'dip'		string	(default "internal,filler")
+'diffopt' 'dip'		string	(default "internal,filler,closeoff")
 			global
 			{not available when compiled without the |+diff|
 			feature}
@@ -2650,6 +2650,12 @@ A jump table for the options with a shor
 		vertical	Start diff mode with vertical splits (unless
 				explicitly specified otherwise).
 
+		closeoff	When a window is closed where 'diff' is set
+				and there is only one window remaining in the
+				same tab page with 'diff' set, execute
+				`:diffoff` in that window.  This undoes a
+				`:diffsplit` command.
+
 		hiddenoff	Do not use diff mode for a buffer when it
 				becomes hidden.
 
@@ -2933,6 +2939,9 @@ A jump table for the options with a shor
 	won't work by default.
 	NOTE: This option is set to the Vi default value when 'compatible' is
 	set and to the Vim default value when 'compatible' is reset.
+	NOTE: when this option is off then the |modifyOtherKeys| functionality
+	is disabled while in Insert mode to avoid ending Insert mode with any
+	key that has a modifier.
 
 						*'eventignore'* *'ei'*
 'eventignore' 'ei'	string	(default "")

--- a/src/diff.c
+++ b/src/diff.c
@@ -35,8 +35,9 @@ static int diff_need_update = FALSE; // 
 #define DIFF_VERTICAL	0x080	// vertical splits
 #define DIFF_HIDDEN_OFF	0x100	// diffoff when hidden
 #define DIFF_INTERNAL	0x200	// use internal xdiff algorithm
+#define DIFF_CLOSE_OFF	0x400	// diffoff when closing window
 #define ALL_WHITE_DIFF (DIFF_IWHITE | DIFF_IWHITEALL | DIFF_IWHITEEOL)
-static int	diff_flags = DIFF_INTERNAL | DIFF_FILLER;
+static int	diff_flags = DIFF_INTERNAL | DIFF_FILLER | DIFF_CLOSE_OFF;
 
 static long diff_algorithm = 0;
 
@@ -1545,6 +1546,14 @@ ex_diffoff(exarg_T *eap)
     if (eap->forceit)
 	diff_buf_clear();
 
+    if (!diffwin)
+    {
+	diff_need_update = FALSE;
+	curtab->tp_diff_invalid = FALSE;
+	curtab->tp_diff_update = FALSE;
+	diff_clear(curtab);
+    }
+
     /* Remove "hor" from from 'scrollopt' if there are no diff windows left. */
     if (!diffwin && vim_strchr(p_sbo, 'h') != NULL)
 	do_cmdline_cmd((char_u *)"set sbo-=hor");
@@ -2222,6 +2231,11 @@ diffopt_changed(void)
 	    p += 9;
 	    diff_flags_new |= DIFF_HIDDEN_OFF;
 	}
+	else if (STRNCMP(p, "closeoff", 8) == 0)
+	{
+	    p += 8;
+	    diff_flags_new |= DIFF_CLOSE_OFF;
+	}
 	else if (STRNCMP(p, "indent-heuristic", 16) == 0)
 	{
 	    p += 16;
@@ -2310,6 +2324,15 @@ diffopt_hiddenoff(void)
 }
 
 /*
+ * Return TRUE if 'diffopt' contains "closeoff".
+ */
+    int
+diffopt_closeoff(void)
+{
+    return (diff_flags & DIFF_CLOSE_OFF) != 0;
+}
+
+/*
  * Find the difference within a changed line.
  * Returns TRUE if the line was added, no other buffer has it.
  */

--- a/src/optiondefs.h
+++ b/src/optiondefs.h
@@ -846,7 +846,8 @@ static struct vimoption options[] =
 								     |P_NODUP,
 #ifdef FEAT_DIFF
 			    (char_u *)&p_dip, PV_NONE,
-			    {(char_u *)"internal,filler", (char_u *)NULL}
+			    {(char_u *)"internal,filler,closeoff",
+								(char_u *)NULL}
 #else
 			    (char_u *)NULL, PV_NONE,
 			    {(char_u *)"", (char_u *)NULL}

--- a/src/proto/diff.pro
+++ b/src/proto/diff.pro
@@ -19,6 +19,7 @@ void diff_set_topline(win_T *fromwin, wi
 int diffopt_changed(void);
 int diffopt_horizontal(void);
 int diffopt_hiddenoff(void);
+int diffopt_closeoff(void);
 int diff_find_change(win_T *wp, linenr_T lnum, int *startp, int *endp);
 int diff_infold(win_T *wp, linenr_T lnum);
 void nv_diffgetput(int put, long count);

--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
@@ -964,3 +964,28 @@ func Test_diff_of_diff()
   call StopVimInTerminal(buf)
   call delete('Xtest_diff_diff')
 endfunc
+
+func CloseoffSetup()
+  enew
+  call setline(1, ['one', 'two', 'three'])
+  diffthis
+  new
+  call setline(1, ['one', 'tow', 'three'])
+  diffthis
+  call assert_equal(1, &diff)
+  only!
+endfunc
+
+func Test_diff_closeoff()
+  " "closeoff" included by default: last diff win gets 'diff' reset'
+  call CloseoffSetup()
+  call assert_equal(0, &diff)
+  enew!
+
+  " "closeoff" excluded: last diff win keeps 'diff' set'
+  set diffopt-=closeoff
+  call CloseoffSetup()
+  call assert_equal(1, &diff)
+  diffoff!
+  enew!
+endfunc

--- a/src/version.c
+++ b/src/version.c
@@ -742,6 +742,8 @@ static char *(features[]) =
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2289,
+/**/
     2288,
 /**/
     2287,

--- a/src/window.c
+++ b/src/window.c
@@ -2438,6 +2438,9 @@ win_close(win_T *win, int free_buf)
     int		help_window = FALSE;
     tabpage_T   *prev_curtab = curtab;
     frame_T	*win_frame = win->w_frame->fr_parent;
+#ifdef FEAT_DIFF
+    int		had_diffmode = win->w_p_diff;
+#endif
 
     if (ERROR_IF_POPUP_WINDOW)
 	return FAIL;
@@ -2625,6 +2628,23 @@ win_close(win_T *win, int free_buf)
     if (help_window)
 	restore_snapshot(SNAP_HELP_IDX, close_curwin);
 
+#ifdef FEAT_DIFF
+    // If the window had 'diff' set and now there is only one window left in
+    // the tab page with 'diff' set, and "closeoff" is in 'diffopt', then
+    // execute ":diffoff!".
+    if (diffopt_closeoff() && had_diffmode && curtab == prev_curtab)
+    {
+	int	diffcount = 0;
+	win_T	*dwin;
+
+	FOR_ALL_WINDOWS(dwin)
+	    if (dwin->w_p_diff)
+		++diffcount;
+	if (diffcount == 1)
+	    do_cmdline_cmd((char_u *)"diffoff!");
+    }
+#endif
+
 #if defined(FEAT_GUI)
     /* When 'guioptions' includes 'L' or 'R' may have to remove scrollbars. */
     if (gui.in_use && !win_hasvertsplit())