Handle the XTMODKEYS escape code

Treat the XTerm specific modifyOtherKeys escape codes as putting the
disambiguate mode on/off the stack. The escape code causes XTerm to use
a (broken, naturally) variant of CSI u encoding for many keys, so we
treat it as going into the disambiguate mode.

docs/keyboard-protocol.rst

@@ -260,6 +260,11 @@ and alternate screens. If a pop request is received that empties the stack,
 all flags are reset. If a push request is received and the stack is full, the
 oldest entry from the stack must be evicted.
 
+.. note:: In the interests of interoperation, the XTerm specific sequences
+   `CSI > 4; 1 m` and `CSI > 4; 0 m` are treated as `CSI > 1 u` and `CSI < 1 u`.
+   These codes cause XTerm to use the CSI u encoding for more keys and are therefore
+   treated as similar to the disambiguate progressive enhancement.
+
 .. _disambiguate:
 
 Disambiguate escape codes

kitty/parser.c

@@ -943,7 +943,7 @@ dispatch_csi(Screen *screen, PyObject DUMP_UNUSED *dump_callback) {
             break;
         case 'm':
             if (start_modifier == '>' && !end_modifier) {
-                REPORT_ERROR("Ignoring xterm specific key modifier resource options (CSI > m)");
+                CALL_CSI_HANDLER2(screen_xtmodkeys, 0, 0);
                 break;
             }
             /* fallthrough */

kitty/screen.c

@@ -886,6 +886,16 @@ screen_pop_key_encoding_flags(Screen *self, uint32_t num) {
     }
 }
 
+void
+screen_xtmodkeys(Screen *self, uint32_t p1, uint32_t p2) {
+    // this is the legacy XTerm escape code for modify keys
+    // https://invisible-island.net/xterm/ctlseqs/ctlseqs.html#h4-Functions-using-CSI-_-ordered-by-the-final-character-lparen-s-rparen:CSI-gt-Pp-m.1DB2
+    // we handle them as being equivalent to push and pop 1 onto the keyboard stack
+    if ((!p1 && !p2) || (p1 == 4 && p2 == 0)) screen_pop_key_encoding_flags(self, 1);
+    else if (p1 == 4 && p2 == 1) screen_push_key_encoding_flags(self, 1);
+}
+
+
 // }}}
 
 // Cursor {{{

kitty/screen.h

@@ -229,6 +229,7 @@ void screen_push_key_encoding_flags(Screen *self, uint32_t val);
 void screen_pop_key_encoding_flags(Screen *self, uint32_t num);
 uint8_t screen_current_key_encoding_flags(Screen *self);
 void screen_report_key_encoding_flags(Screen *self);
+void screen_xtmodkeys(Screen *self, uint32_t p1, uint32_t p2);
 #define DECLARE_CH_SCREEN_HANDLER(name) void screen_##name(Screen *screen);
 DECLARE_CH_SCREEN_HANDLER(bell)
 DECLARE_CH_SCREEN_HANDLER(backspace)