feat(terminal): respond to OSC background and foreground request (#17197)

The motivation for this update is Issue #15365, where background=light
is not properly set for Nvim running from an Nvim :terminal. This can be
encountered when e.g., opening a terminal to make git commits, which
opens EDITOR=nvim in the nested terminal.

Under the implementation of this commit, the OSC response always
indicates a black or white foreground/background. While this may not
reflect the actual foreground/background color, it permits 'background'
to be retained for a nested Nvim instance running in the terminal
emulator. The behaviour matches Vim.

5 files changed, 137 insertions, 106 deletions

diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index cb70c81191..7743f5981a 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -287,6 +287,8 @@ The following new APIs and features were added.
 • Terminal buffers emit a |TermRequest| autocommand event when the child
   process emits an OSC or DCS control sequence.
 
+• Terminal buffers respond to OSC background and foreground requests. |default-autocmds|
+
 ==============================================================================
 CHANGED FEATURES                                                 *news-changed*
 
diff --git a/runtime/doc/vim_diff.txt b/runtime/doc/vim_diff.txt
index 279fdd646f..46550f42b7 100644
--- a/runtime/doc/vim_diff.txt
+++ b/runtime/doc/vim_diff.txt
@@ -145,6 +145,12 @@ nvim_terminal:
 - BufReadCmd: Treats "term://" buffers as |terminal| buffers. |terminal-start|
 - TermClose: A |terminal| buffer started with no arguments (which thus uses
   'shell') and which exits with no error is closed automatically.
+- TermRequest: The terminal emulator responds to OSC background and foreground
+  requests, indicating (1) a black background and white foreground when Nvim
+  option 'background' is "dark" or (2) a white background and black foreground
+  when 'background' is "light". While this may not reflect the actual
+  foreground/background color, it permits 'background' to be retained for a
+  nested Nvim instance running in the terminal emulator.
 
 nvim_cmdwin:
 - CmdwinEnter: Limits syntax sync to maxlines=1 in the |cmdwin|.
diff --git a/runtime/lua/vim/_defaults.lua b/runtime/lua/vim/_defaults.lua
index 64eb638fd7..07850a5a47 100644
--- a/runtime/lua/vim/_defaults.lua
+++ b/runtime/lua/vim/_defaults.lua
@@ -155,6 +155,30 @@ do
     end,
   })
 
+  vim.api.nvim_create_autocmd('TermRequest', {
+    group = nvim_terminal_augroup,
+    desc = 'Respond to OSC foreground/background color requests',
+    callback = function(args)
+      local fg_request = args.data == '\027]10;?'
+      local bg_request = args.data == '\027]11;?'
+      if fg_request or bg_request then
+        -- WARN: This does not return the actual foreground/background color,
+        -- but rather returns:
+        --   - fg=white/bg=black when Nvim option 'background' is 'dark'
+        --   - fg=black/bg=white when Nvim option 'background' is 'light'
+        local red, green, blue = 0, 0, 0
+        local bg_option_dark = vim.o.background == 'dark'
+        if (fg_request and bg_option_dark) or (bg_request and not bg_option_dark) then
+          red, green, blue = 65535, 65535, 65535
+        end
+        local command = fg_request and 10 or 11
+        local data = string.format('\027]%d;rgb:%04x/%04x/%04x\007', command, red, green, blue)
+        local channel = vim.bo[args.buf].channel
+        vim.api.nvim_chan_send(channel, data)
+      end
+    end,
+  })
+
   vim.api.nvim_create_autocmd('CmdwinEnter', {
     pattern = '[:>]',
     desc = 'Limit syntax sync to maxlines=1 in the command window',
diff --git a/test/functional/core/startup_spec.lua b/test/functional/core/startup_spec.lua
index fd3748a985..f4a9c0c8d7 100644
--- a/test/functional/core/startup_spec.lua
+++ b/test/functional/core/startup_spec.lua
@@ -1369,12 +1369,19 @@ describe('inccommand on ex mode', function()
     local screen
     screen = Screen.new(60, 10)
     screen:attach()
-    local id = fn.termopen(
-      { nvim_prog, '-u', 'NONE', '-c', 'set termguicolors', '-E', 'test/README.md' },
-      {
-        env = { VIMRUNTIME = os.getenv('VIMRUNTIME') },
-      }
-    )
+    local id = fn.termopen({
+      nvim_prog,
+      '-u',
+      'NONE',
+      '-i',
+      'NONE',
+      '-c',
+      'set termguicolors background=dark',
+      '-E',
+      'test/README.md',
+    }, {
+      env = { VIMRUNTIME = os.getenv('VIMRUNTIME') },
+    })
     fn.chansend(id, '%s/N')
     screen:expect {
       grid = [[
diff --git a/test/functional/terminal/tui_spec.lua b/test/functional/terminal/tui_spec.lua
index 54fbfd191a..46d3c98e07 100644
--- a/test/functional/terminal/tui_spec.lua
+++ b/test/functional/terminal/tui_spec.lua
@@ -8,6 +8,7 @@ local helpers = require('test.functional.helpers')(after_each)
 local thelpers = require('test.functional.terminal.helpers')
 local Screen = require('test.functional.ui.screen')
 local eq = helpers.eq
+local feed_command = helpers.feed_command
 local feed_data = thelpers.feed_data
 local clear = helpers.clear
 local command = helpers.command
@@ -2126,7 +2127,7 @@ describe('TUI FocusGained/FocusLost', function()
       '--cmd',
       'colorscheme vim',
       '--cmd',
-      'set noswapfile noshowcmd noruler notermguicolors',
+      'set noswapfile noshowcmd noruler notermguicolors background=dark',
     })
 
     screen:expect([[
@@ -2776,10 +2777,73 @@ describe('TUI', function()
 end)
 
 describe('TUI bg color', function()
-  local screen
+  before_each(clear)
 
-  local function setup_bg_test()
-    clear()
+  local attr_ids = {
+    [1] = { reverse = true },
+    [2] = { bold = true },
+    [3] = { reverse = true, bold = true },
+    [4] = { foreground = tonumber('0x00000a') },
+  }
+
+  it('is properly set in a nested Nvim instance when background=dark', function()
+    command('highlight clear Normal')
+    command('set background=dark') -- set outer Nvim background
+    local screen = thelpers.setup_child_nvim({
+      '-u',
+      'NONE',
+      '-i',
+      'NONE',
+      '--cmd',
+      'colorscheme vim',
+      '--cmd',
+      'set noswapfile',
+    })
+    screen:set_default_attr_ids(attr_ids)
+    retry(nil, 30000, function() -- wait for automatic background processing
+      screen:sleep(20)
+      feed_command('set background?') -- check nested Nvim background
+      screen:expect([[
+      {1: }                                                 |
+      {2:~}                                                 |
+      {2:~}                                                 |
+      {2:~}                                                 |
+      {3:[No Name]                       0,0-1          All}|
+        background=dark                                 |
+      {4:-- TERMINAL --}                                    |
+      ]])
+    end)
+  end)
+
+  it('is properly set in a nested Nvim instance when background=light', function()
+    command('highlight clear Normal')
+    command('set background=light') -- set outer Nvim background
+    local screen = thelpers.setup_child_nvim({
+      '-u',
+      'NONE',
+      '-i',
+      'NONE',
+      '--cmd',
+      'colorscheme vim',
+      '--cmd',
+      'set noswapfile',
+    })
+    retry(nil, 30000, function() -- wait for automatic background processing
+      screen:sleep(20)
+      feed_command('set background?') -- check nested Nvim background
+      screen:expect([[
+      {1: }                                                 |
+      {3:~}                                                 |
+      {3:~}                                                 |
+      {3:~}                                                 |
+      {5:[No Name]                       0,0-1          All}|
+        background=light                                |
+      {3:-- TERMINAL --}                                    |
+      ]])
+    end)
+  end)
+
+  it('queries the terminal for background color', function()
     exec_lua([[
       vim.api.nvim_create_autocmd('TermRequest', {
         callback = function(args)
@@ -2791,8 +2855,7 @@ describe('TUI bg color', function()
         end,
       })
     ]])
-
-    screen = thelpers.setup_child_nvim({
+    thelpers.setup_child_nvim({
       '-u',
       'NONE',
       '-i',
@@ -2800,109 +2863,38 @@ describe('TUI bg color', function()
       '--cmd',
       'colorscheme vim',
       '--cmd',
-      'set noswapfile notermguicolors',
-      '-c',
-      'autocmd OptionSet background echo "did OptionSet, yay!"',
+      'set noswapfile',
     })
-  end
-
-  before_each(setup_bg_test)
-
-  it('queries the terminal for background color', function()
     retry(nil, 1000, function()
       eq(true, eval("get(g:, 'oscrequest', v:false)"))
     end)
   end)
 
-  it('triggers OptionSet event on unsplit terminal-response', function()
-    screen:expect([[
-      {1: }                                                 |
-      {4:~                                                 }|*3
-      {5:[No Name]                       0,0-1          All}|
-                                                        |
-      {3:-- TERMINAL --}                                    |
-    ]])
-    feed_data('\027]11;rgb:ffff/ffff/ffff\027\\')
-    screen:expect { any = 'did OptionSet, yay!' }
-
-    feed_data(':echo "new_bg=".&background\n')
-    screen:expect { any = 'new_bg=light' }
-
-    setup_bg_test()
-    screen:expect([[
-      {1: }                                                 |
-      {4:~                                                 }|*3
-      {5:[No Name]                       0,0-1          All}|
-                                                        |
-      {3:-- TERMINAL --}                                    |
-    ]])
-    feed_data('\027]11;rgba:ffff/ffff/ffff/8000\027\\')
-    screen:expect { any = 'did OptionSet, yay!' }
-
-    feed_data(':echo "new_bg=".&background\n')
-    screen:expect { any = 'new_bg=light' }
-  end)
-
-  it('triggers OptionSet event with split terminal-response', function()
-    screen:expect([[
-      {1: }                                                 |
-      {4:~                                                 }|*3
-      {5:[No Name]                       0,0-1          All}|
-                                                        |
-      {3:-- TERMINAL --}                                    |
-    ]])
-    -- Send a background response with the OSC command part split.
-    feed_data('\027]11;rgb')
-    feed_data(':ffff/ffff/ffff\027\\')
-    screen:expect { any = 'did OptionSet, yay!' }
-
-    feed_data(':echo "new_bg=".&background\n')
-    screen:expect { any = 'new_bg=light' }
-
-    setup_bg_test()
-    screen:expect([[
-      {1: }                                                 |
-      {4:~                                                 }|*3
-      {5:[No Name]                       0,0-1          All}|
-                                                        |
-      {3:-- TERMINAL --}                                    |
-    ]])
-    -- Send a background response with the Pt portion split.
-    feed_data('\027]11;rgba:ffff/fff')
-    feed_data('f/ffff/8000\027\\')
-    screen:expect { any = 'did OptionSet, yay!' }
-
-    feed_data(':echo "new_bg=".&background\n')
-    screen:expect { any = 'new_bg=light' }
-  end)
-
-  it('not triggers OptionSet event with invalid terminal-response', function()
-    screen:expect([[
-      {1: }                                                 |
-      {4:~                                                 }|*3
-      {5:[No Name]                       0,0-1          All}|
-                                                        |
-      {3:-- TERMINAL --}                                    |
-    ]])
-    feed_data('\027]11;rgb:ffff/ffff/ffff/8000\027\\')
-    screen:expect_unchanged()
-
-    feed_data(':echo "new_bg=".&background\n')
-    screen:expect { any = 'new_bg=dark' }
-
-    setup_bg_test()
-    screen:expect([[
+  it('triggers OptionSet from automatic background processing', function()
+    local screen = thelpers.setup_child_nvim({
+      '-u',
+      'NONE',
+      '-i',
+      'NONE',
+      '--cmd',
+      'colorscheme vim',
+      '--cmd',
+      'set noswapfile',
+      '-c',
+      'autocmd OptionSet background echo "did OptionSet, yay!"',
+    })
+    retry(nil, 30000, function() -- wait for automatic background processing
+      screen:sleep(20)
+      screen:expect([[
       {1: }                                                 |
-      {4:~                                                 }|*3
+      {3:~}                                                 |
+      {3:~}                                                 |
+      {3:~}                                                 |
       {5:[No Name]                       0,0-1          All}|
-                                                        |
+      did OptionSet, yay!                               |
       {3:-- TERMINAL --}                                    |
-    ]])
-    feed_data('\027]11;rgba:ffff/foo/ffff/8000\027\\')
-    screen:expect_unchanged()
-
-    feed_data(':echo "new_bg=".&background\n')
-    screen:expect { any = 'new_bg=dark' }
+      ]])
+    end)
   end)
 end)