feat(defaults): enable 'termguicolors' by default when supported by terminal

Enable 'termguicolors' automatically when Nvim can detect that truecolor
is supported by the host terminal.

If $COLORTERM is set to "truecolor" or "24bit", or the terminal's
terminfo entry contains capabilities for Tc, RGB, or setrgbf and
setrgbb, then we assume that the terminal supports truecolor. Otherwise,
the terminal is queried (using both XTGETTCAP and SGR + DECRQSS). If the
terminal's response to these queries (if any) indicates that it supports
truecolor, then 'termguicolors' is enabled.

runtime/doc/news.txt

@@ -98,6 +98,9 @@ The following changes may require adaptations in user config or plugins.
 • Default color scheme has been updated to be "Neovim branded" and accessible.
   Use `:colorscheme vim` to revert to the old legacy color scheme.
 
+• 'termguicolors' is enabled by default when Nvim is able to determine that
+  the host terminal emulator supports 24-bit color.
+
 ==============================================================================
 BREAKING CHANGES IN HEAD                                    *news-breaking-dev*
 

runtime/doc/options.txt

@@ -6495,6 +6495,10 @@ A jump table for the options with a short description can be found at |Q_op|.
 	attributes instead of "cterm" attributes. |guifg|
 	Requires an ISO-8613-3 compatible terminal.
 
+	Nvim will automatically attempt to determine if the host terminal
+	supports 24-bit color and will enable this option if it does
+	(unless explicitly disabled by the user).
+
 					*'termpastefilter'* *'tpf'*
 'termpastefilter' 'tpf'	string	(default "BS,HT,ESC,DEL")
 			global

runtime/doc/vim_diff.txt

@@ -75,6 +75,8 @@ Defaults					            *nvim-defaults*
 - 'switchbuf' defaults to "uselast"
 - 'tabpagemax' defaults to 50
 - 'tags' defaults to "./tags;,tags"
+- 'termguicolors' is enabled by default if Nvim can detect support from the
+  host terminal
 - 'ttimeoutlen' defaults to 50
 - 'ttyfast' is always set
 - 'undodir' defaults to ~/.local/state/nvim/undo// (|xdg|), auto-created

runtime/lua/vim/_defaults.lua

@@ -165,91 +165,92 @@ do
   })
 end
 
---- Guess value of 'background' based on terminal color.
+-- Only do the following when the TUI is attached
----
+local tty = nil
---- We write Operating System Command (OSC) 11 to the terminal to request the
+for _, ui in ipairs(vim.api.nvim_list_uis()) do
---- terminal's background color. We then wait for a response. If the response
+  if ui.chan == 1 and ui.stdout_tty then
---- matches `rgba:RRRR/GGGG/BBBB/AAAA` where R, G, B, and A are hex digits, then
+    tty = ui
---- compute the luminance[1] of the RGB color and classify it as light/dark
+    break
---- accordingly. Note that the color components may have anywhere from one to
---- four hex digits, and require scaling accordingly as values out of 4, 8, 12,
---- or 16 bits. Also note the A(lpha) component is optional, and is parsed but
---- ignored in the calculations.
----
---- [1] https://en.wikipedia.org/wiki/Luma_%28video%29
-do
-  --- Parse a string of hex characters as a color.
-  ---
-  --- The string can contain 1 to 4 hex characters. The returned value is
-  --- between 0.0 and 1.0 (inclusive) representing the intensity of the color.
-  ---
-  --- For instance, if only a single hex char "a" is used, then this function
-  --- returns 0.625 (10 / 16), while a value of "aa" would return 0.664 (170 /
-  --- 256).
-  ---
-  --- @param c string Color as a string of hex chars
-  --- @return number? Intensity of the color
-  local function parsecolor(c)
-    if #c == 0 or #c > 4 then
-      return nil
-    end
-
-    local val = tonumber(c, 16)
-    if not val then
-      return nil
-    end
-
-    local max = tonumber(string.rep('f', #c), 16)
-    return val / max
   end
+end
 
-  --- Parse an OSC 11 response
+if tty then
+  --- Guess value of 'background' based on terminal color.
   ---
-  --- Either of the two formats below are accepted:
+  --- We write Operating System Command (OSC) 11 to the terminal to request the
+  --- terminal's background color. We then wait for a response. If the response
+  --- matches `rgba:RRRR/GGGG/BBBB/AAAA` where R, G, B, and A are hex digits, then
+  --- compute the luminance[1] of the RGB color and classify it as light/dark
+  --- accordingly. Note that the color components may have anywhere from one to
+  --- four hex digits, and require scaling accordingly as values out of 4, 8, 12,
+  --- or 16 bits. Also note the A(lpha) component is optional, and is parsed but
+  --- ignored in the calculations.
   ---
-  ---   OSC 11 ; rgb:<red>/<green>/<blue>
+  --- [1] https://en.wikipedia.org/wiki/Luma_%28video%29
-  ---
+  do
-  --- or
+    --- Parse a string of hex characters as a color.
-  ---
+    ---
-  ---   OSC 11 ; rgba:<red>/<green>/<blue>/<alpha>
+    --- The string can contain 1 to 4 hex characters. The returned value is
-  ---
+    --- between 0.0 and 1.0 (inclusive) representing the intensity of the color.
-  --- where
+    ---
-  ---
+    --- For instance, if only a single hex char "a" is used, then this function
-  ---   <red>, <green>, <blue>, <alpha> := h | hh | hhh | hhhh
+    --- returns 0.625 (10 / 16), while a value of "aa" would return 0.664 (170 /
-  ---
+    --- 256).
-  --- The alpha component is ignored, if present.
+    ---
-  ---
+    --- @param c string Color as a string of hex chars
-  --- @param resp string OSC 11 response
+    --- @return number? Intensity of the color
-  --- @return string? Red component
+    local function parsecolor(c)
-  --- @return string? Green component
+      if #c == 0 or #c > 4 then
-  --- @return string? Blue component
+        return nil
-  local function parseosc11(resp)
-    local r, g, b
-    r, g, b = resp:match('^\027%]11;rgb:(%x+)/(%x+)/(%x+)$')
-    if not r and not g and not b then
-      local a
-      r, g, b, a = resp:match('^\027%]11;rgba:(%x+)/(%x+)/(%x+)/(%x+)$')
-      if not a or #a > 4 then
-        return nil, nil, nil
       end
+
+      local val = tonumber(c, 16)
+      if not val then
+        return nil
+      end
+
+      local max = tonumber(string.rep('f', #c), 16)
+      return val / max
     end
 
-    if r and g and b and #r <= 4 and #g <= 4 and #b <= 4 then
+    --- Parse an OSC 11 response
-      return r, g, b
+    ---
+    --- Either of the two formats below are accepted:
+    ---
+    ---   OSC 11 ; rgb:<red>/<green>/<blue>
+    ---
+    --- or
+    ---
+    ---   OSC 11 ; rgba:<red>/<green>/<blue>/<alpha>
+    ---
+    --- where
+    ---
+    ---   <red>, <green>, <blue>, <alpha> := h | hh | hhh | hhhh
+    ---
+    --- The alpha component is ignored, if present.
+    ---
+    --- @param resp string OSC 11 response
+    --- @return string? Red component
+    --- @return string? Green component
+    --- @return string? Blue component
+    local function parseosc11(resp)
+      local r, g, b
+      r, g, b = resp:match('^\027%]11;rgb:(%x+)/(%x+)/(%x+)$')
+      if not r and not g and not b then
+        local a
+        r, g, b, a = resp:match('^\027%]11;rgba:(%x+)/(%x+)/(%x+)/(%x+)$')
+        if not a or #a > 4 then
+          return nil, nil, nil
+        end
+      end
+
+      if r and g and b and #r <= 4 and #g <= 4 and #b <= 4 then
+        return r, g, b
+      end
+
+      return nil, nil, nil
     end
 
-    return nil, nil, nil
-  end
-
-  local tty = false
-  for _, ui in ipairs(vim.api.nvim_list_uis()) do
-    if ui.chan == 1 and ui.stdout_tty then
-      tty = true
-      break
-    end
-  end
-
-  if tty then
     local timer = assert(vim.uv.new_timer())
 
     ---@param bg string New value of the 'background' option
@@ -300,7 +301,7 @@ do
     io.stdout:write('\027]11;?\007')
 
     timer:start(1000, 0, function()
-      -- No response received. Delete the autocommand
+      -- Delete the autocommand if no response was received
       vim.schedule(function()
         -- Suppress error if autocommand has already been deleted
         pcall(vim.api.nvim_del_autocmd, id)
@@ -311,4 +312,108 @@ do
       end
     end)
   end
+
+  --- If the TUI (term_has_truecolor) was able to determine that the host
+  --- terminal supports truecolor, enable 'termguicolors'. Otherwise, query the
+  --- terminal (using both XTGETTCAP and SGR + DECRQSS). If the terminal's
+  --- response indicates that it does support truecolor enable 'termguicolors',
+  --- but only if the user has not already disabled it.
+  do
+    if tty.rgb then
+      -- The TUI was able to determine truecolor support
+      vim.o.termguicolors = true
+    else
+      --- Enable 'termguicolors', but only if it was not already set by the user.
+      local function settgc()
+        if not vim.api.nvim_get_option_info2('termguicolors', {}).was_set then
+          vim.o.termguicolors = true
+        end
+      end
+
+      local caps = {} ---@type table<string, boolean>
+      require('vim.termcap').query({ 'Tc', 'RGB', 'setrgbf', 'setrgbb' }, function(cap, found)
+        if not found then
+          return
+        end
+
+        caps[cap] = true
+        if caps.Tc or caps.RGB or (caps.setrgbf and caps.setrgbb) then
+          settgc()
+        end
+      end)
+
+      local timer = assert(vim.uv.new_timer())
+
+      -- Arbitrary colors to set in the SGR sequence
+      local r = 1
+      local g = 2
+      local b = 3
+
+      local id = vim.api.nvim_create_autocmd('TermResponse', {
+        nested = true,
+        callback = function(args)
+          local resp = args.data ---@type string
+          local decrqss = resp:match('^\027P1%$r([%d;:]+)m$')
+
+          if decrqss then
+            -- The DECRQSS SGR response first contains attributes separated by
+            -- semicolons, followed by the SGR itself with parameters separated
+            -- by colons. Some terminals include "0" in the attribute list
+            -- unconditionally; others do not. Our SGR sequence did not set any
+            -- 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
+            end
+
+            -- The returned SGR sequence should begin with 48:2
+            local sgr = attrs[#attrs]:match('^48:2:([%d:]+)$')
+            if not sgr then
+              return true
+            end
+
+            -- The remaining elements of the SGR sequence should be the 3 colors
+            -- we set. Some terminals also include an additional parameter
+            -- (which can even be empty!), so handle those cases as well
+            local params = vim.split(sgr, ':')
+            if #params ~= 3 and (#params ~= 4 or (params[1] ~= '' and params[1] ~= '1')) then
+              return true
+            end
+
+            if
+              tonumber(params[#params - 2]) == r
+              and tonumber(params[#params - 1]) == g
+              and tonumber(params[#params]) == b
+            then
+              settgc()
+            end
+
+            return true
+          end
+        end,
+      })
+
+      -- Write SGR followed by DECRQSS. This sets the background color then
+      -- immediately asks the terminal what the background color is. If the
+      -- terminal responds to the DECRQSS with the same SGR sequence that we
+      -- sent then the terminal supports truecolor.
+      local decrqss = '\027P$qm\027\\'
+      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))
+
+      timer:start(1000, 0, function()
+        -- Delete the autocommand if no response was received
+        vim.schedule(function()
+          -- Suppress error if autocommand has already been deleted
+          pcall(vim.api.nvim_del_autocmd, id)
+        end)
+
+        if not timer:is_closing() then
+          timer:close()
+        end
+      end)
+    end
+  end
 end

runtime/lua/vim/_meta.lua

@@ -20,6 +20,7 @@ vim.lsp = require('vim.lsp')
 vim.re = require('vim.re')
 vim.secure = require('vim.secure')
 vim.snippet = require('vim.snippet')
+vim.text = require('vim.text')
 vim.treesitter = require('vim.treesitter')
 vim.ui = require('vim.ui')
 vim.version = require('vim.version')

runtime/lua/vim/_meta/options.lua

@@ -6941,6 +6941,10 @@ vim.go.tbidi = vim.go.termbidi
 --- attributes instead of "cterm" attributes. `guifg`
 --- Requires an ISO-8613-3 compatible terminal.
 ---
+--- Nvim will automatically attempt to determine if the host terminal
+--- supports 24-bit color and will enable this option if it does
+--- (unless explicitly disabled by the user).
+---
 --- @type boolean
 vim.o.termguicolors = false
 vim.o.tgc = vim.o.termguicolors

runtime/lua/vim/termcap.lua

@@ -12,7 +12,10 @@ local M = {}
 --- emulator supports the XTGETTCAP sequence.
 ---
 --- @param caps string|table A terminal capability or list of capabilities to query
---- @param cb function(cap:string, seq:string) Function to call when a response is received
+--- @param cb function(cap:string, found:bool, seq:string?) Callback function which is called for
+---           each capability in {caps}. {found} is set to true if the capability was found or false
+---           otherwise. {seq} is the control sequence for the capability if found, or nil for
+---           boolean capabilities.
 function M.query(caps, cb)
   vim.validate({
     caps = { caps, { 'string', 'table' } },
@@ -23,21 +26,33 @@ function M.query(caps, cb)
     caps = { caps }
   end
 
-  local count = #caps
+  local pending = {} ---@type table<string, boolean>
+  for _, v in ipairs(caps) do
+    pending[v] = true
+  end
 
-  vim.api.nvim_create_autocmd('TermResponse', {
+  local timer = assert(vim.uv.new_timer())
+
+  local id = vim.api.nvim_create_autocmd('TermResponse', {
     callback = function(args)
       local resp = args.data ---@type string
-      local k, v = resp:match('^\027P1%+r(%x+)=(%x+)$')
+      local k, rest = resp:match('^\027P1%+r(%x+)(.*)$')
-      if k and v then
+      if k and rest then
         local cap = vim.text.hexdecode(k)
-        local seq =
+        local seq ---@type string?
-          vim.text.hexdecode(v):gsub('\\E', '\027'):gsub('%%p%d', ''):gsub('\\(%d+)', string.char)
+        if rest:match('^=%x+$') then
+          seq = vim.text
+            .hexdecode(rest:sub(2))
+            :gsub('\\E', '\027')
+            :gsub('%%p%d', '')
+            :gsub('\\(%d+)', string.char)
+        end
 
-        cb(cap, seq)
+        cb(cap, true, seq)
 
-        count = count - 1
+        pending[cap] = nil
-        if count == 0 then
+
+        if next(pending) == nil then
           return true
         end
       end
@@ -57,6 +72,23 @@ function M.query(caps, cb)
   end
 
   io.stdout:write(query)
+
+  timer:start(1000, 0, function()
+    -- Delete the autocommand if no response was received
+    vim.schedule(function()
+      -- Suppress error if autocommand has already been deleted
+      pcall(vim.api.nvim_del_autocmd, id)
+
+      -- Call the callback for all capabilities that were not found
+      for k in pairs(pending) do
+        cb(k, false, nil)
+      end
+    end)
+
+    if not timer:is_closing() then
+      timer:close()
+    end
+  end)
 end
 
 return M

runtime/plugin/osc52.lua

@@ -6,7 +6,11 @@ if not tty or vim.g.clipboard ~= nil or vim.o.clipboard ~= '' or not os.getenv('
   return
 end
 
-require('vim.termcap').query('Ms', function(cap, seq)
+require('vim.termcap').query('Ms', function(cap, found, seq)
+  if not found then
+    return
+  end
+
   assert(cap == 'Ms')
 
   -- Check 'clipboard' and g:clipboard again to avoid a race condition
@@ -16,7 +20,7 @@ require('vim.termcap').query('Ms', function(cap, seq)
 
   -- If the terminal reports a sequence other than OSC 52 for the Ms capability
   -- then ignore it. We only support OSC 52 (for now)
-  if not seq:match('^\027%]52') then
+  if not seq or not seq:match('^\027%]52') then
     return
   end
 

src/nvim/options.lua

@@ -8764,6 +8764,10 @@ return {
         Enables 24-bit RGB color in the |TUI|.  Uses "gui" |:highlight|
         attributes instead of "cterm" attributes. |guifg|
         Requires an ISO-8613-3 compatible terminal.
+
+        Nvim will automatically attempt to determine if the host terminal
+        supports 24-bit color and will enable this option if it does
+        (unless explicitly disabled by the user).
       ]=],
       full_name = 'termguicolors',
       redraw = { 'ui_option' },

src/nvim/tui/tui.c

@@ -148,7 +148,8 @@ static bool cursor_style_enabled = false;
 # include "tui/tui.c.generated.h"
 #endif
 
-void tui_start(TUIData **tui_p, int *width, int *height, char **term)
+void tui_start(TUIData **tui_p, int *width, int *height, char **term, bool *rgb)
+  FUNC_ATTR_NONNULL_ALL
 {
   TUIData *tui = xcalloc(1, sizeof(TUIData));
   tui->is_starting = true;
@@ -177,6 +178,7 @@ void tui_start(TUIData **tui_p, int *width, int *height, char **term)
   *width = tui->width;
   *height = tui->height;
   *term = tui->term;
+  *rgb = tui->rgb;
 }
 
 void tui_set_key_encoding(TUIData *tui)
@@ -334,6 +336,9 @@ static void terminfo_start(TUIData *tui)
   int konsolev = konsolev_env ? (int)strtol(konsolev_env, NULL, 10)
                               : (konsole ? 1 : 0);
 
+  // truecolor support must be checked before patching/augmenting terminfo
+  tui->rgb = term_has_truecolor(tui, colorterm);
+
   patch_terminfo_bugs(tui, term, colorterm, vtev, konsolev, iterm_env, nsterm);
   augment_terminfo(tui, term, vtev, konsolev, iterm_env, nsterm);
   tui->can_change_scroll_region =
@@ -1439,7 +1444,7 @@ void tui_suspend(TUIData *tui)
     tui_mouse_on(tui);
   }
   stream_set_blocking(tui->input.in_fd, false);  // libuv expects this
-  ui_client_attach(tui->width, tui->height, tui->term);
+  ui_client_attach(tui->width, tui->height, tui->term, tui->rgb);
 #endif
 }
 
@@ -1752,6 +1757,44 @@ static int unibi_find_ext_bool(unibi_term *ut, const char *name)
   return -1;
 }
 
+/// Determine if the terminal supports truecolor or not:
+///
+/// 1. If $COLORTERM is "24bit" or "truecolor", return true
+/// 2. Else, check terminfo for Tc, RGB, setrgbf, or setrgbb capabilities. If
+///    found, return true
+/// 3. Else, return false
+static bool term_has_truecolor(TUIData *tui, const char *colorterm)
+{
+  // Check $COLORTERM
+  if (strequal(colorterm, "truecolor") || strequal(colorterm, "24bit")) {
+    return true;
+  }
+
+  // Check for Tc and RGB
+  for (size_t i = 0; i < unibi_count_ext_bool(tui->ut); i++) {
+    const char *n = unibi_get_ext_bool_name(tui->ut, i);
+    if (n && (!strcmp(n, "Tc") || !strcmp(n, "RGB"))) {
+      return true;
+    }
+  }
+
+  // Check for setrgbf and setrgbb
+  bool setrgbf = false;
+  bool setrgbb = false;
+  for (size_t i = 0; i < unibi_count_ext_str(tui->ut) && (!setrgbf || !setrgbb); i++) {
+    const char *n = unibi_get_ext_str_name(tui->ut, i);
+    if (n) {
+      if (!setrgbf && !strcmp(n, "setrgbf")) {
+        setrgbf = true;
+      } else if (!setrgbb && !strcmp(n, "setrgbb")) {
+        setrgbb = true;
+      }
+    }
+  }
+
+  return setrgbf && setrgbb;
+}
+
 /// Patches the terminfo records after loading from system or built-in db.
 /// Several entries in terminfo are known to be deficient or outright wrong;
 /// and several terminal emulators falsely announce incorrect terminal types.

src/nvim/ui_client.c

@@ -70,14 +70,14 @@ uint64_t ui_client_start_server(int argc, char **argv)
   return channel->id;
 }
 
-void ui_client_attach(int width, int height, char *term)
+void ui_client_attach(int width, int height, char *term, bool rgb)
 {
   MAXSIZE_TEMP_ARRAY(args, 3);
   ADD_C(args, INTEGER_OBJ(width));
   ADD_C(args, INTEGER_OBJ(height));
 
   MAXSIZE_TEMP_DICT(opts, 9);
-  PUT_C(opts, "rgb", BOOLEAN_OBJ(true));
+  PUT_C(opts, "rgb", BOOLEAN_OBJ(rgb));
   PUT_C(opts, "ext_linegrid", BOOLEAN_OBJ(true));
   PUT_C(opts, "ext_termcolors", BOOLEAN_OBJ(true));
   if (term) {
@@ -111,9 +111,10 @@ void ui_client_run(bool remote_ui)
   ui_client_is_remote = remote_ui;
   int width, height;
   char *term;
-  tui_start(&tui, &width, &height, &term);
+  bool rgb;
+  tui_start(&tui, &width, &height, &term, &rgb);
 
-  ui_client_attach(width, height, term);
+  ui_client_attach(width, height, term, rgb);
 
   // os_exit() will be invoked when the client channel detaches
   while (true) {