Allow clients to query if the terminal emulator support styled underlines

kitty/modes.h

@@ -76,5 +76,8 @@
 #define BRACKETED_PASTE_START "\033[200~"
 #define BRACKETED_PASTE_END  "\033[201~"
 
+// Styled underlines
+#define STYLED_UNDERLINES (2016 << 5)
+
 // Extended keyboard protocol
 #define EXTENDED_KEYBOARD (2017 << 5)

kitty/screen.c

@@ -923,6 +923,8 @@ report_mode_status(Screen *self, unsigned int which, bool private) {
         KNOWN_MODE(EXTENDED_KEYBOARD);
         KNOWN_MODE(FOCUS_TRACKING);
 #undef KNOWN_MODE
+        case STYLED_UNDERLINES:
+            ans = 3; break;
     }
     if (snprintf(buf, sizeof(buf) - 1, "\x1b[%s%u;%u$y", (private ? "?" : ""), which, ans)) callback("write_to_child", self, buf, 0);
 }

kitty_tests/parser.py

@@ -139,6 +139,9 @@ class TestParser(BaseTest):
         pb('\033[?1l', ('screen_reset_mode', 1, 1))
         self.assertFalse(s.cursor_key_mode)
         c.clear()
+        pb('\033[?2016$p', ('report_mode_status', 2016, 1))
+        self.ae(c.wtcbuf, b'\033[?2016;3$y')
+        c.clear()
         pb('\033[?1$p', ('report_mode_status', 1, 1))
         self.ae(c.wtcbuf, b'\033[?1;2$y')
         pb('\033[2;4r', ('screen_set_margins', 2, 4))

protocol-extensions.asciidoc

@@ -44,6 +44,23 @@ To reset the underline color (also previously reserved and unused):
 <ESC>[59m
 ```
 
+A client can query if the terminal emulator supports this underline extension using the 
+standard link:http://vt100.net/docs/vt510-rm/DECRQM[DECRQM] escape sequence, with the 
+mode value `2016`, like this:
+
+Client sends:
+
+```
+<ESC>[?2016$p
+```
+
+If the terminal supports this underline extension, it must respond with
+link:http://vt100.net/docs/vt510-rm/DECRPM[DECRPM]
+
+```
+<ESC>[?2016;3$p
+```
+
 
 == Graphics rendering