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

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.
----
---- 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.
----
---- [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
+-- Only do the following when the TUI is attached
+local tty = nil
+for _, ui in ipairs(vim.api.nvim_list_uis()) do
+  if ui.chan == 1 and ui.stdout_tty then
+    tty = ui
+    break
   end
+end
 
-  --- Parse an OSC 11 response
-  ---
-  --- Either of the two formats below are accepted:
-  ---
-  ---   OSC 11 ; rgb:<red>/<green>/<blue>
-  ---
-  --- or
-  ---
-  ---   OSC 11 ; rgba:<red>/<green>/<blue>/<alpha>
+if tty then
+  --- Guess value of 'background' based on terminal color.
   ---
-  --- where
+  --- 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.
   ---
-  ---   <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
+  --- [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
-    end
 
-    if r and g and b and #r <= 4 and #g <= 4 and #b <= 4 then
-      return r, g, b
+      local val = tonumber(c, 16)
+      if not val then
+        return nil
+      end
+
+      local max = tonumber(string.rep('f', #c), 16)
+      return val / max
     end
 
-    return nil, nil, nil
-  end
+    --- Parse an OSC 11 response
+    ---
+    --- 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
 
-  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
+      return nil, nil, nil
     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')