patch 8.2.4504: when there is a partially matching map full map may not work

Problem:    When there is a partially matching map and modifyOtherKeys is
            active a full map may not work.
Solution:   Only simplify modifiers when there is no matching mapping.
            (closes #8792)

src/getchar.c

@@ -2598,7 +2598,7 @@ handle_mapping(
 	}
 
 	// If no partly match found, use the longest full match.
-	if (keylen != KEYLEN_PART_MAP)
+	if (keylen != KEYLEN_PART_MAP && mp_match != NULL)
 	{
 	    mp = mp_match;
 	    keylen = mp_match_len;
@@ -2643,7 +2643,7 @@ handle_mapping(
 	    max_mlen = mlen + 1;
     }
 
-    if ((mp == NULL || max_mlen >= mp_match_len) && keylen != KEYLEN_PART_MAP)
+    if ((mp == NULL || max_mlen > mp_match_len) && keylen != KEYLEN_PART_MAP)
     {
 	int	save_keylen = keylen;
 

src/testdir/test_termcodes.vim

@@ -2098,6 +2098,23 @@ func Test_modifyOtherKeys_mapped()
   set timeoutlen&
 endfunc
 
+func Test_modifyOtherKeys_ambiguous_mapping()
+  new
+  set timeoutlen=10
+  map <C-J> a
+  map <C-J>x <Nop>
+  call setline(1, 'x')
+
+  " CTRL-J b should have trigger the <C-J> mapping and then insert "b"
+  call feedkeys(GetEscCodeCSI27('J', 5) .. "b\<Esc>", 'Lx!')
+  call assert_equal('xb', getline(1))
+
+  unmap <C-J>
+  unmap <C-J>x
+  set timeoutlen&
+  bwipe!
+endfunc
+
 " Whether Shift-Tab sends "ESC [ Z" or "ESC [ 27 ; 2 ; 9 ~" is unpredictable,
 " both should work.
 func Test_modifyOtherKeys_shift_tab()

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4504,
 /**/
     4503,
 /**/