feat(tui): query extended underline support using DECRQSS (#28052)

runtime/lua/vim/_defaults.lua

@@ -384,13 +384,13 @@ if tty then
             -- attributes, so there should be no attributes in the list.
             local attrs = vim.split(decrqss, ';')
             if #attrs ~= 1 and (#attrs ~= 2 or attrs[1] ~= '0') then
-              return true
+              return false
             end
 
             -- The returned SGR sequence should begin with 48:2
             local sgr = attrs[#attrs]:match('^48:2:([%d:]+)$')
             if not sgr then
-              return true
+              return false
             end
 
             -- The remaining elements of the SGR sequence should be the 3 colors
@@ -422,7 +422,8 @@ if tty then
       if os.getenv('TMUX') then
         decrqss = string.format('\027Ptmux;%s\027\\', decrqss:gsub('\027', '\027\027'))
       end
-      io.stdout:write(string.format('\027[48;2;%d;%d;%dm%s', r, g, b, decrqss))
+      -- Reset attributes first, as other code may have set attributes.
+      io.stdout:write(string.format('\027[0m\027[48;2;%d;%d;%dm%s', r, g, b, decrqss))
 
       timer:start(1000, 0, function()
         -- Delete the autocommand if no response was received

src/nvim/tui/input.c

@@ -553,6 +553,13 @@ static void handle_term_response(TermInput *input, const TermKeyKey *key)
   if (termkey_interpret_string(input->tk, key, &str) == TERMKEY_RES_KEY) {
     assert(str != NULL);
 
+    // Handle DECRQSS SGR response for the query from tui_query_extended_underline().
+    // Some terminals include "0" in the attribute list unconditionally; others don't.
+    if (key->type == TERMKEY_TYPE_DCS
+        && (strnequal(str, S_LEN("1$r4:3m")) || strnequal(str, S_LEN("1$r0;4:3m")))) {
+      tui_enable_extended_underline(input->tui_data);
+    }
+
     // Send an event to nvim core. This will update the v:termresponse variable
     // and fire the TermResponse event
     MAXSIZE_TEMP_ARRAY(args, 2);

src/nvim/tui/tui.c

@@ -225,6 +225,26 @@ void tui_handle_term_mode(TUIData *tui, TermMode mode, TermModeState state)
   }
 }
 
+/// Query the terminal emulator to see if it supports extended underline.
+static void tui_query_extended_underline(TUIData *tui)
+{
+  // Try to set an undercurl using an SGR sequence, followed by a DECRQSS SGR query.
+  // Reset attributes first, as other code may have set attributes.
+  out(tui, S_LEN("\x1b[0m\x1b[4:3m\x1bP$qm\x1b\\"));
+  tui->print_attr_id = -1;
+}
+
+void tui_enable_extended_underline(TUIData *tui)
+{
+  if (tui->unibi_ext.set_underline_style == -1) {
+    tui->unibi_ext.set_underline_style = (int)unibi_add_ext_str(tui->ut, "ext.set_underline_style",
+                                                                "\x1b[4:%p1%dm");
+  }
+  // Only support colon syntax. #9270
+  tui->unibi_ext.set_underline_color = (int)unibi_add_ext_str(tui->ut, "ext.set_underline_color",
+                                                              "\x1b[58:2::%p1%d:%p2%d:%p3%dm");
+}
+
 /// Query the terminal emulator to see if it supports Kitty's keyboard protocol.
 ///
 /// Write CSI ? u followed by a primary device attributes request (CSI c). If
@@ -306,6 +326,7 @@ static void terminfo_start(TUIData *tui)
   tui->unibi_ext.reset_scroll_region = -1;
   tui->unibi_ext.set_cursor_style = -1;
   tui->unibi_ext.reset_cursor_style = -1;
+  tui->unibi_ext.set_underline_style = -1;
   tui->unibi_ext.set_underline_color = -1;
   tui->unibi_ext.sync = -1;
   tui->out_fd = STDOUT_FILENO;
@@ -390,6 +411,11 @@ static void terminfo_start(TUIData *tui)
   // mode 2026
   tui_request_term_mode(tui, kTermModeSynchronizedOutput);
 
+  if (tui->unibi_ext.set_underline_style == -1) {
+    // Query the terminal to see if it supports extended underline.
+    tui_query_extended_underline(tui);
+  }
+
   // Query the terminal to see if it supports Kitty's keyboard protocol
   tui_query_kitty_keyboard(tui);
 
@@ -2331,14 +2357,10 @@ static void augment_terminfo(TUIData *tui, const char *term, int vte_version, in
     if (vte_version >= 5102 || konsolev >= 221170
         || (ext_bool_Su != -1 && unibi_get_ext_bool(ut, (size_t)ext_bool_Su))
         || (weztermv != NULL && strcmp(weztermv, "20210203-095643") > 0)) {
-      tui->unibi_ext.set_underline_style = (int)unibi_add_ext_str(ut, "ext.set_underline_style",
+      tui_enable_extended_underline(tui);
-                                                                  "\x1b[4:%p1%dm");
     }
-  }
+  } else {
-  if (tui->unibi_ext.set_underline_style != -1) {
+    tui_enable_extended_underline(tui);
-    // Only support colon syntax. #9270
-    tui->unibi_ext.set_underline_color = (int)unibi_add_ext_str(ut, "ext.set_underline_color",
-                                                                "\x1b[58:2::%p1%d:%p2%d:%p3%dm");
   }
 
   if (!kitty && (vte_version == 0 || vte_version >= 5400)) {

test/functional/terminal/tui_spec.lua

@@ -1588,6 +1588,35 @@ describe('TUI', function()
     }
   end)
 
+  -- Note: libvterm doesn't support colored underline or undercurl.
+  it('supports undercurl and underdouble when run in :terminal', function()
+    screen:set_default_attr_ids({
+      [1] = { reverse = true },
+      [2] = { bold = true, reverse = true },
+      [3] = { bold = true },
+      [4] = { foreground = 12 },
+      [5] = { undercurl = true },
+      [6] = { underdouble = true },
+    })
+    child_session:request('nvim_set_hl', 0, 'Visual', { undercurl = true })
+    feed_data('ifoobar\027V')
+    screen:expect([[
+      {5:fooba}{1:r}                                            |
+      {4:~                                                 }|*3
+      {2:[No Name] [+]                                     }|
+      {3:-- VISUAL LINE --}                                 |
+      {3:-- TERMINAL --}                                    |
+    ]])
+    child_session:request('nvim_set_hl', 0, 'Visual', { underdouble = true })
+    screen:expect([[
+      {6:fooba}{1:r}                                            |
+      {4:~                                                 }|*3
+      {2:[No Name] [+]                                     }|
+      {3:-- VISUAL LINE --}                                 |
+      {3:-- TERMINAL --}                                    |
+    ]])
+  end)
+
   it('in nvim_list_uis()', function()
     -- $TERM in :terminal.
     local exp_term = is_os('bsd') and 'builtin_xterm' or 'xterm-256color'