UI: always use contrete colors for default_colors_set

But add an escape hatch needed for external TUI, so it still can use
terminal emulator defaults.

runtime/doc/ui.txt

@@ -35,6 +35,7 @@ a dictionary with these (optional) keys:
 	`ext_linegrid`		Use new revision of the grid events. |ui-linegrid|
 	`ext_multigrid`		Use per-window grid based events. |ui-multigrid|
 	`ext_hlstate`		Use detailed highlight state. |ui-hlstate|
+	`ext_termcolors`	Use external default colors.
 
 Specifying a non-existent option is an error. UIs can check the |api-metadata|
 `ui_options` key for supported options. Additionally Nvim (currently) requires
@@ -239,6 +240,13 @@ numerical highlight `id`:s to the actual attributes.
 	special colors respectively. `cterm_fg` and `cterm_bg` specifies the
 	default color codes to use in a 256-color terminal.
 
+	The rgb values will always be valid colors, by default. If no
+	colors have been set, they will default to black and white, depending
+	on 'background'. By setting the `ext_termcolors` option, instead
+	-1 will be used for unset colors. This is mostly useful for a
+	TUI implementation, where using the terminal emulators builitn
+	defaults are expected.
+
 	Note: unlike the corresponding events in the first revision, the
 	screen is not always cleared after sending this event. The GUI has to
 	repaint the screen with changed background color itself.

src/nvim/api/ui.c

@@ -210,8 +210,9 @@ static void ui_set_option(UI *ui, bool init, String name, Object value,
       return;
     }
     ui->rgb = value.data.boolean;
-    // A little drastic, but only legacy uis need to use this option
+    // A little drastic, but only takes effect for legacy uis. For linegrid UI
-    if (!init) {
+    // only changes metadata for nvim_list_uis(), no refresh needed.
+    if (!init && !ui->ui_ext[kUILinegrid]) {
       ui_refresh();
     }
     return;
@@ -355,6 +356,12 @@ static void remote_ui_default_colors_set(UI *ui, Integer rgb_fg,
                                          Integer rgb_bg, Integer rgb_sp,
                                          Integer cterm_fg, Integer cterm_bg)
 {
+  if (!ui->ui_ext[kUITermColors]) {
+    bool dark = (*p_bg == 'd');
+    rgb_fg = rgb_fg != -1 ? rgb_fg : (dark ? 0xFFFFFF : 0x000000);
+    rgb_bg = rgb_bg != -1 ? rgb_bg : (dark ? 0x000000 : 0xFFFFFF);
+    rgb_sp = rgb_sp != -1 ? rgb_sp : 0xFF0000;
+  }
   Array args = ARRAY_DICT_INIT;
   ADD(args, INTEGER_OBJ(rgb_fg));
   ADD(args, INTEGER_OBJ(rgb_bg));

src/nvim/ui.c

@@ -310,6 +310,9 @@ void ui_set_ext_option(UI *ui, UIExtension ext, bool active)
     ui->option_set(ui, cstr_as_string((char *)ui_ext_names[ext]),
                    BOOLEAN_OBJ(active));
   }
+  if (ext == kUITermColors) {
+    ui_default_colors_set();
+  }
 }
 
 void ui_line(ScreenGrid *grid, int row, int startcol, int endcol, int clearcol,

src/nvim/ui.h

@@ -18,6 +18,7 @@ typedef enum {
   kUILinegrid,
   kUIMultigrid,
   kUIHlState,
+  kUITermColors,
   kUIExtCount,
 } UIExtension;
 
@@ -29,6 +30,7 @@ EXTERN const char *ui_ext_names[] INIT(= {
   "ext_linegrid",
   "ext_multigrid",
   "ext_hlstate",
+  "ext_termcolors",
 });
 
 

test/functional/api/vim_spec.lua

@@ -1278,6 +1278,7 @@ describe('API', function()
           ext_linegrid = screen._options.ext_linegrid or false,
           ext_multigrid = false,
           ext_hlstate=false,
+          ext_termcolors=false,
           height = 4,
           rgb = true,
           width = 20,

test/functional/terminal/tui_spec.lua

@@ -256,13 +256,13 @@ describe('TUI', function()
   end)
 
   it('shows up in nvim_list_uis', function()
-    feed_data(':echo map(nvim_list_uis(), {k,v -> sort(items(v))})\013')
+    feed_data(':echo map(nvim_list_uis(), {k,v -> sort(items(filter(v, {k,v -> k[:3] !=# "ext_" })))})\013')
     screen:expect([=[
-      [[['ext_cmdline', v:false], ['ext_hlstate', v:fals|
+                                                        |
-      e], ['ext_linegrid', v:true], ['ext_multigrid', v:|
+      {4:~                                                 }|
-      false], ['ext_popupmenu', v:false], ['ext_tabline'|
+      {5:                                                  }|
-      , v:false], ['ext_wildmenu', v:false], ['height', |
+      [[['height', 6], ['rgb', v:false], ['width', 50]]]|
-      6], ['rgb', v:false], ['width', 50]]]             |
+                                                        |
       {10:Press ENTER or type command to continue}{1: }          |
       {3:-- TERMINAL --}                                    |
     ]=])

test/functional/ui/options_spec.lua

@@ -27,6 +27,7 @@ describe('ui receives option updates', function()
       ext_linegrid=false,
       ext_hlstate=false,
       ext_multigrid=false,
+      ext_termcolors=false,
     }
 
     clear(...)

test/functional/ui/screen_basic_spec.lua

@@ -960,3 +960,46 @@ end)
 describe("Screen (line-based)", function()
   screen_tests(true)
 end)
+
+describe('Screen default colors', function()
+  local screen
+  local function startup(light, termcolors)
+    local extra = (light and ' background=light') or ''
+
+    local nvim_argv = {helpers.nvim_prog, '-u', 'NONE', '-i', 'NONE', '-N',
+                       '--cmd', 'set shortmess+=I noswapfile belloff= noshowcmd noruler'..extra,
+                       '--embed'}
+    local screen_nvim = spawn(nvim_argv)
+    set_session(screen_nvim)
+    screen = Screen.new()
+    screen:attach(termcolors and {rgb=true,ext_termcolors=true} or {rgb=true})
+  end
+
+  it('are dark per default', function()
+    startup(false, false)
+    screen:expect{condition=function()
+      eq({rgb_bg=0, rgb_fg=Screen.colors.White, rgb_sp=Screen.colors.Red,
+          cterm_bg=0, cterm_fg=0}, screen.default_colors)
+    end}
+  end)
+
+  it('can be set to light', function()
+    startup(true, false)
+    screen:expect{condition=function()
+      eq({rgb_bg=Screen.colors.White, rgb_fg=0, rgb_sp=Screen.colors.Red,
+          cterm_bg=0, cterm_fg=0}, screen.default_colors)
+    end}
+  end)
+
+  it('can be handled by external terminal', function()
+    startup(false, true)
+    screen:expect{condition=function()
+      eq({rgb_bg=-1, rgb_fg=-1, rgb_sp=-1, cterm_bg=0, cterm_fg=0}, screen.default_colors)
+    end}
+
+    startup(true, true)
+    screen:expect{condition=function()
+      eq({rgb_bg=-1, rgb_fg=-1, rgb_sp=-1, cterm_bg=0, cterm_fg=0}, screen.default_colors)
+    end}
+  end)
+end)