patch 8.2.1930: wrong input if removing shift results in special key code

Problem:    Wrong input if removing shift results in special key code.
Solution:   Handle special key codes. (closes #7189)

src/term.c

@@ -4462,7 +4462,8 @@ modifiers2keycode(int modifiers, int *key, char_u *string)
     if (modifiers != 0)
     {
 	// Some keys have the modifier included.  Need to handle that here to
-	// make mappings work.
+	// make mappings work.  This may result in a special key, such as
+	// K_S_TAB.
 	*key = simplify_key(*key, &modifiers);
 	if (modifiers != 0)
 	{
@@ -4793,7 +4794,13 @@ handle_key_with_modifier(
     // insert modifiers with KS_MODIFIER
     new_slen = modifiers2keycode(modifiers, &key, string);
 
-    if (has_mbyte)
+    if (IS_SPECIAL(key))
+    {
+	string[new_slen++] = K_SPECIAL;
+	string[new_slen++] = KEY2TERMCAP0(key);
+	string[new_slen++] = KEY2TERMCAP1(key);
+    }
+    else if (has_mbyte)
 	new_slen += (*mb_char2bytes)(key, string + new_slen);
     else
 	string[new_slen++] = key;

src/testdir/test_termcodes.vim

@@ -2029,6 +2029,23 @@ func Test_modifyOtherKeys_mapped()
   set timeoutlen&
 endfunc
 
+" Whether Shift-Tab sends "ESC [ Z" or "ESC [ 27 ; 2 ; 9 ~" is unpredictable,
+" both should work.
+func Test_modifyOtherKeys_shift_tab()
+  set timeoutlen=10
+
+  call setline(1, '')
+  call feedkeys("a\<C-K>" .. GetEscCodeCSI27("\t", '2') .. "\<Esc>", 'Lx!')
+  eval getline(1)->assert_equal('<S-Tab>')
+
+  call setline(1, '')
+  call feedkeys("a\<C-K>\<Esc>[Z\<Esc>", 'Lx!')
+  eval getline(1)->assert_equal('<S-Tab>')
+
+  set timeoutlen&
+  bwipe!
+endfunc
+
 func RunTest_mapping_works_with_shift(func)
   new
   set timeoutlen=10

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1930,
 /**/
     1929,
 /**/