patch 9.0.0973: Kitty keyboard protocol key with NumLock not decoded

Problem:    Kitty keyboard protocol key not decoded when it has an unsupported
            modifier, such as NumLock.
Solution:   Accept a key with any modifier. (closes #11638)

src/term.c

@@ -4693,6 +4693,8 @@ decode_modifiers(int n)
 	modifiers |= MOD_MASK_CTRL;
     if (code & 8)
 	modifiers |= MOD_MASK_META;
+    // Any further modifiers are silently dropped.
+
     return modifiers;
 }
 
@@ -5317,14 +5319,14 @@ handle_csi(
     // Key with modifier:
     //	{lead}27;{modifier};{key}~
     //	{lead}{key};{modifier}u
-    // Only handles four modifiers, this won't work if the modifier value is
-    // more than 16.
-    else if (((arg[0] == 27 && argc == 3 && trail == '~')
+    // Even though we only handle four modifiers and the {modifier} value
+    // should be 16 or lower, we accept all modifier values to avoid the raw
+    // sequence to be passed through.
+    else if ((arg[0] == 27 && argc == 3 && trail == '~')
 		|| (argc == 2 && trail == 'u'))
-	    && arg[1] <= 16)
     {
 	return len + handle_key_with_modifier(arg, trail,
-			    csi_len, offset, buf, bufsize, buflen);
+					csi_len, offset, buf, bufsize, buflen);
     }
 
     // Key without modifier (Kitty sends this for Esc):

src/testdir/test_termcodes.vim

@@ -2459,6 +2459,30 @@ func Test_mapping_works_with_shift_ctrl_alt()
   call RunTest_mapping_works_with_mods(function('GetEscCodeCSIu'), 'C-S-A', 8)
 endfunc
 
+func Test_mapping_works_with_unknown_modifiers()
+  new
+  set timeoutlen=10
+
+  for Func in [function('GetEscCodeCSI27'), function('GetEscCodeCSIu')]
+    call RunTest_mapping_mods('<C-z>', 'z', Func, 5)
+    " Add 16, 32, 64 or 128 for modifiers we currently don't support.
+    call RunTest_mapping_mods('<C-z>', 'z', Func, 5 + 16)
+    call RunTest_mapping_mods('<C-z>', 'z', Func, 5 + 32)
+    call RunTest_mapping_mods('<C-z>', 'z', Func, 5 + 64)
+    call RunTest_mapping_mods('<C-z>', 'z', Func, 5 + 128)
+
+    call RunTest_mapping_mods('<S-X>', 'X', Func, 2)
+    " Add 16, 32, 64 or 128 for modifiers we currently don't support.
+    call RunTest_mapping_mods('<S-X>', 'X', Func, 2 + 16)
+    call RunTest_mapping_mods('<S-X>', 'X', Func, 2 + 32)
+    call RunTest_mapping_mods('<S-X>', 'X', Func, 2 + 64)
+    call RunTest_mapping_mods('<S-X>', 'X', Func, 2 + 128)
+  endfor
+
+  bwipe!
+  set timeoutlen&
+endfunc
+
 func Test_insert_literal()
   set timeoutlen=10
 

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    973,
 /**/
     972,
 /**/