docs: deprecate the "term_background" UI field

runtime/doc/deprecated.txt

@@ -202,6 +202,8 @@ UI EXTENSIONS
 			- `["wildmenu_show", items]`
 			- `["wildmenu_select", selected]`
 			- `["wildmenu_hide"]`
+- *term_background*	Unused. The terminal background color is now detected
+			by the Nvim core directly instead of the TUI.
 
 VARIABLES
 - *b:terminal_job_pid*	PID of the top-level process in a |:terminal|.

runtime/doc/news.txt

@@ -340,4 +340,8 @@ release.
 • vim.treesitter.languagetree functions:
   - |LanguageTree:for_each_child()|	Use |LanguageTree:children()| (non-recursive) instead.
 
+• The "term_background" UI option |ui-ext-options| is deprecated and no longer
+  populated. Background color detection is now performed in Lua by the Nvim
+  core, not the TUI.
+
  vim:tw=78:ts=8:sw=2:et:ft=help:norl:

runtime/doc/ui.txt

@@ -52,7 +52,6 @@ with these (optional) keys:
 - `ext_termcolors`	Use external default colors.
 - `term_name`		Sets the name of the terminal 'term'.
 - `term_colors`		Sets the number of supported colors 't_Co'.
-- `term_background`	Sets the default value of 'background'.
 - `stdin_fd`		Read buffer 1 from this fd as if it were stdin |--|.
 			Only from |--embed| UI on startup. |ui-startup-stdin|
 - `stdin_tty`		Tells if `stdin` is a `tty` or not.

src/nvim/api/ui.c

@@ -120,7 +120,6 @@ void remote_ui_disconnect(uint64_t channel_id)
 
   // Destroy `ui`.
   XFREE_CLEAR(ui->term_name);
-  XFREE_CLEAR(ui->term_background);
   xfree(ui);
 }
 

src/nvim/ui.c

@@ -623,7 +623,10 @@ Array ui_array(void)
 
     // TUI fields. (`stdin_fd` is intentionally omitted.)
     PUT(info, "term_name", CSTR_TO_OBJ(ui->term_name));
-    PUT(info, "term_background", CSTR_TO_OBJ(ui->term_background));
+
+    // term_background is deprecated. Populate with an empty string
+    PUT(info, "term_background", CSTR_TO_OBJ(""));
+
     PUT(info, "term_colors", INTEGER_OBJ(ui->term_colors));
     PUT(info, "stdin_tty", BOOLEAN_OBJ(ui->stdin_tty));
     PUT(info, "stdout_tty", BOOLEAN_OBJ(ui->stdout_tty));

src/nvim/ui.h

@@ -104,7 +104,8 @@ struct ui_t {
 
   // TUI fields.
   char *term_name;
-  char *term_background;
+  char *term_background;  ///< Deprecated. No longer needed since background color detection happens
+                          ///< in Lua. To be removed in a future release.
   int term_colors;
   bool stdin_tty;
   bool stdout_tty;

src/nvim/ui_client.c

@@ -84,10 +84,7 @@ void ui_client_attach(int width, int height, char *term)
   if (term) {
     PUT_C(opts, "term_name", CSTR_AS_OBJ(term));
   }
-  if (ui_client_bg_response != kNone) {
-    bool is_dark = (ui_client_bg_response == kTrue);
-    PUT_C(opts, "term_background", CSTR_AS_OBJ(is_dark ? "dark" : "light"));
-  }
+
   PUT_C(opts, "term_colors", INTEGER_OBJ(t_colors));
   if (!ui_client_is_remote) {
     PUT_C(opts, "stdin_tty", BOOLEAN_OBJ(stdin_isatty));

src/nvim/ui_client.h

@@ -31,10 +31,6 @@ EXTERN int ui_client_exit_status INIT( = 0);
 /// Whether ui client has sent nvim_ui_attach yet
 EXTERN bool ui_client_attached INIT( = false);
 
-/// Whether ui client has gotten a response about the bg color of the terminal,
-/// kTrue=dark, kFalse=light, kNone=no response yet
-EXTERN TriState ui_client_bg_response INIT( = kNone);
-
 /// The ui client should forward its stdin to the nvim process
 /// by convention, this uses fd=3 (next free number after stdio)
 EXTERN bool ui_client_forward_stdin INIT( = false);

test/functional/api/ui_spec.lua

@@ -36,8 +36,6 @@ describe('nvim_ui_attach()', function()
       pcall_err(meths.ui_attach, 80, 24, { term_name=true }))
     eq("Invalid 'term_colors': expected Integer, got Boolean",
       pcall_err(meths.ui_attach, 80, 24, { term_colors=true }))
-    eq("Invalid 'term_background': expected String, got Boolean",
-      pcall_err(meths.ui_attach, 80, 24, { term_background=true }))
     eq("Invalid 'stdin_fd': expected Integer, got String",
       pcall_err(meths.ui_attach, 80, 24, { stdin_fd='foo' }))
     eq("Invalid 'stdin_tty': expected Boolean, got String",

test/functional/ui/options_spec.lua

@@ -210,22 +210,6 @@ describe('UI can set terminal option', function()
     screen = Screen.new(20,5)
   end)
 
-  it('term_background', function()
-    eq('dark', eval '&background')
-
-    screen:attach {term_background='light'}
-    eq('light', eval '&background')
-  end)
-
-  it("term_background but not if 'background' already set by user", function()
-    eq('dark', eval '&background')
-    command 'set background=dark'
-
-    screen:attach {term_background='light'}
-
-    eq('dark', eval '&background')
-  end)
-
   it('term_name', function()
     eq('nvim', eval '&term')