Change undercurl escape code to 4:x

See https://github.com/neovim/neovim/issues/7479
for discussion. Also implement double underline.

kitty/cell_vertex.glsl

@@ -62,6 +62,7 @@ const uint ZERO = uint(0);
 const uint ONE = uint(1);
 const uint TWO = uint(2);
 const uint THREE = uint(3);
+const uint FOUR = uint(4);
 const uint DECORATION_MASK = uint(3);
 const uint STRIKE_MASK = uint(1);
 const uint REVERSE_MASK = uint(1);
@@ -159,8 +160,8 @@ void main() {
     // Underline and strike through (rendered via sprites)
     float in_url = in_range(c, r);
     decoration_fg = choose_color(in_url, color_to_vec(url_color), to_color(colors[2], resolved_fg));
-    underline_pos = choose_color(in_url, to_sprite_pos(pos, TWO, ZERO, ZERO), to_sprite_pos(pos, (text_attrs >> 2) & DECORATION_MASK, ZERO, ZERO));
-    strike_pos = to_sprite_pos(pos, ((text_attrs >> 7) & STRIKE_MASK) * THREE, ZERO, ZERO);
+    underline_pos = choose_color(in_url, to_sprite_pos(pos, THREE, ZERO, ZERO), to_sprite_pos(pos, (text_attrs >> 2) & DECORATION_MASK, ZERO, ZERO));
+    strike_pos = to_sprite_pos(pos, ((text_attrs >> 7) & STRIKE_MASK) * FOUR, ZERO, ZERO);
 
     // Cursor
     foreground = choose_color(cursor, bg, foreground);

kitty/cursor.c

@@ -80,9 +80,9 @@ cursor_from_sgr(Cursor *self, unsigned int *params, unsigned int count) {
             case 3:
                 self->italic = true;  break;
             case 4:
-                self->decoration = 1;  break;
-            case UNDERCURL_CODE:
-                self->decoration = 2;  break;
+                if (i < count) { self->decoration = MIN(3, params[i]); i++; }
+                else self->decoration = 1;
+                break;
             case 7:
                 self->reverse = true;  break;
             case 9:
@@ -143,7 +143,8 @@ static inline const char*
 decoration_as_sgr(uint8_t decoration) {
     switch(decoration) {
         case 1: return "4"; 
-        case 2: return "6";  // UNDERCURL_CODE
+        case 2: return "4:2";  
+        case 3: return "4:3";
         default: return "24";
     }
 }

kitty/data-types.h

@@ -58,7 +58,6 @@ typedef enum MouseShapes { BEAM, HAND, ARROW } MouseShape;
 #define CC_SHIFT 16
 #define UTF8_ACCEPT 0
 #define UTF8_REJECT 1
-#define UNDERCURL_CODE 6
 #define DECORATION_FG_CODE 58
 #define CHAR_IS_BLANK(ch) ((ch) == 32 || (ch) == 0)
 #define CONTINUED_MASK 1

kitty/fonts/render.py

@@ -74,12 +74,17 @@ def add_line(buf, cell_width, position, thickness, cell_height):
     y = position - thickness // 2
     while thickness:
         thickness -= 1
-        offset = cell_width * y
-        for x in range(cell_width):
-            buf[offset + x] = 255
+        ctypes.memset(ctypes.addressof(buf) + (cell_width * y), 255, cell_width)
         y += 1
 
 
+def add_dline(buf, cell_width, position, thickness, cell_height):
+    bottom = min(position - thickness, cell_height - 1)
+    top = min(position, cell_height - 1)
+    for y in {top, bottom}:
+        ctypes.memset(ctypes.addressof(buf) + (cell_width * y), 255, cell_width)
+
+
 def add_curl(buf, cell_width, position, thickness, cell_height):
     xfactor = 2.0 * pi / cell_width
     yfactor = thickness
@@ -119,12 +124,9 @@ def render_special(underline=0, strikethrough=False, missing=False):
 
     if underline:
         t = underline_thickness
-        if underline == 2:
+        if underline > 1:
             t = max(1, min(cell_height - underline_position - 1, t))
-        dl(
-            add_curl
-            if underline == 2 else add_line, underline_position, t, cell_height
-        )
+        dl([None, add_line, add_dline, add_curl][underline], underline_position, t, cell_height)
     if strikethrough:
         pos = int(0.65 * baseline)
         dl(add_line, pos, underline_thickness, cell_height)
@@ -138,7 +140,7 @@ def render_special(underline=0, strikethrough=False, missing=False):
 
 def prerender():
     # Pre-render the special blank, underline and strikethrough cells
-    cells = render_special(1), render_special(2), render_special(0, True), render_special(missing=True)
+    cells = render_special(1), render_special(2), render_special(3), render_special(0, True), render_special(missing=True)
     if send_prerendered_sprites(*map(ctypes.addressof, cells)) != len(cells):
         raise RuntimeError('Your GPU has too small a max texture size')
 

protocol-extensions.asciidoc

@@ -28,11 +28,15 @@ in terminal editors such as vim and emacs to display red, wavy underlines under
 mis-spelled words and/or syntax errors. This is done by re-purposing some SGR escape codes
 that are not used in modern terminals (https://en.wikipedia.org/wiki/ANSI_escape_code#CSI_codes)
 
-To change the underline style from straight line to curl (this used to be the
-code for rapid blinking text, only previous use I know of was in MS-DOS ANSI.sys):
+To set the underline style:
 
 ```
-<ESC>[6m
+<ESC>[4:0m  # this is no underline
+<ESC>[4:1m  # this is a straight underline
+<ESC>[4:2m  # this is a double underline
+<ESC>[4:3m  # this is a curly underline
+<ESC>[4m    # this is a straight underline (for backwards compat)
+<ESC>[24m   # this is no underline (for backwards compat)
 ```
 
 To set the underline color (this is reserved and as far as I can tell not actually used for anything):