kitty: initial support for “report alternate key”

In this mode, the “shifted” and “base layout” keys are added to the
CSIs, as sub-parameters to the “key” parameter.

Note that this PR only implements the “shifted” key, not the “base
layout key”.

This is done by converting the original XKB symbol to it’s
corresponding UTF-32 codepoint. If this codepoint is different from
the one we use as “key” in the CSI, we add it as a sub-parameter.

Related to #319

CHANGELOG.md

@@ -44,6 +44,9 @@
 * Kitty keyboard protocol:
   - [Report event types](https://sw.kovidgoyal.net/kitty/keyboard-protocol/#report-events)
     (mode `0b10`)
+  - [Report alternate keys](https://sw.kovidgoyal.net/kitty/keyboard-protocol/#report-alternates)
+    (mode `0b100`, but not that only the _shifted_ key is reported,
+    not the _base layout key_)
   - [Report all keys as escape codes](https://sw.kovidgoyal.net/kitty/keyboard-protocol/#report-all-keys)
     (mode `0b1000`)
   - [Report associated text](https://sw.kovidgoyal.net/kitty/keyboard-protocol/#report-text)

input.c

@@ -1161,6 +1161,7 @@ kitty_kbd_protocol(struct seat *seat, struct terminal *term,
 
     const bool disambiguate = flags & KITTY_KBD_DISAMBIGUATE;
     const bool report_events = flags & KITTY_KBD_REPORT_EVENT;
+    const bool report_alternate = flags & KITTY_KBD_REPORT_ALTERNATE;
     const bool report_all_as_escapes = flags & KITTY_KBD_REPORT_ALL;
 
     if (!report_events && released)
@@ -1252,7 +1253,7 @@ emit_escapes:
         encoded_mods |= mods & (1 << seat->kbd.mod_num)   ? (1 << 7) : 0;
     encoded_mods++;
 
-    int key = -1;
+    int key = -1, alternate = -1;
     char final;
 
     switch (sym) {
@@ -1435,6 +1436,8 @@ emit_escapes:
             key = xkb_keysym_to_utf32(sym_to_use);
             if (key == 0)
                 key = sym_to_use;
+            else
+                alternate = xkb_keysym_to_utf32(sym);
         }
 
         final = 'u';
@@ -1465,6 +1468,11 @@ emit_escapes:
         bytes = snprintf(p, left, "\x1b[%u", key);
         p += bytes; left -= bytes;
 
+        if (report_alternate && alternate > 0 && alternate != key) {
+            bytes = snprintf(p, left, ":%u", alternate);
+            p += bytes; left -= bytes;
+        }
+
         if (encoded_mods > 1 || event[0] != '\0' || report_associated_text) {
             bytes = snprintf(p, left, ";%u%s", encoded_mods, event);
             p += bytes; left -= bytes;

terminal.h

@@ -135,6 +135,7 @@ enum kitty_kbd_flags {
     KITTY_KBD_REPORT_ASSOCIATED = 0x10,
     KITTY_KBD_SUPPORTED = (KITTY_KBD_DISAMBIGUATE |
                            KITTY_KBD_REPORT_EVENT |
+                           KITTY_KBD_REPORT_ALTERNATE |
                            KITTY_KBD_REPORT_ALL |
                            KITTY_KBD_REPORT_ASSOCIATED),
 };