TUI: rework background-color detection

- Like Vim, use set_option_value() followed by reset_option_was_set().
- Do not use set_string_default(), so the default is predictable.
  This affects `:set bg&`.
- Wait until end-of-startup (VimEnter) to handle the response. The
  response is racey anyways, so timing is irrelevant. This allows
  OptionSet to be triggered, unlike during startup.

src/nvim/globals.h

@@ -83,10 +83,10 @@ EXTERN struct nvim_stats_s {
   int64_t redraw;
 } g_stats INIT(= { 0, 0 });
 
-/* Values for "starting" */
-#define NO_SCREEN       2       /* no screen updating yet */
-#define NO_BUFFERS      1       /* not all buffers loaded yet */
-/*                      0          not starting anymore */
+// Values for "starting".
+#define NO_SCREEN       2       // no screen updating yet
+#define NO_BUFFERS      1       // not all buffers loaded yet
+//                      0          not starting anymore
 
 /*
  * Number of Rows and Columns in the screen.

src/nvim/option.c

@@ -883,7 +883,7 @@ set_options_default (
 /// @param name The name of the option
 /// @param val The value of the option
 /// @param allocated If true, do not copy default as it was already allocated.
-void set_string_default(const char *name, char *val, bool allocated)
+static void set_string_default(const char *name, char *val, bool allocated)
   FUNC_ATTR_NONNULL_ALL
 {
   int opt_idx = findoption(name);

src/nvim/tui/input.c

@@ -357,12 +357,28 @@ static bool handle_forced_escape(TermInput *input)
 static void set_bg_deferred(void **argv)
 {
   char *bgvalue = argv[0];
-  set_string_default("bg", bgvalue, false);
-  if (!option_was_set("bg")) {
-    set_option_value("bg", 0, bgvalue, 0);
+  if (starting) {
+    // Wait until after startup, so OptionSet is triggered.
+    loop_schedule(&main_loop, event_create(set_bg_deferred, 1, bgvalue));
+    return;
+  }
+  if (!option_was_set("bg") && !strequal((char *)p_bg, bgvalue)) {
+    // Value differs, apply it.
+    set_option_value("bg", 0L, bgvalue, 0);
+    reset_option_was_set("bg");
   }
 }
 
+// During startup, tui.c requests the background color (see `ext.get_bg`).
+//
+// Here in input.c, we watch for the terminal response `\e]11;COLOR\a`.  If
+// COLOR matches `rgb:RRRR/GGGG/BBBB` where R, G, and B 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.
+//
+// [1] https://en.wikipedia.org/wiki/Luma_%28video%29
 static bool handle_background_color(TermInput *input)
 {
   size_t count = 0;
@@ -407,7 +423,10 @@ static bool handle_background_color(TermInput *input)
       double b = (double)rgb[2] / (double)rgb_max[2];
       double luminance = (0.299 * r) + (0.587 * g) + (0.114 * b);  // CCIR 601
       char *bgvalue = luminance < 0.5 ? "dark" : "light";
+      DLOG("bg response: %s", bgvalue);
       loop_schedule(&main_loop, event_create(set_bg_deferred, 1, bgvalue));
+    } else {
+      DLOG("failed to parse bg response");
     }
     return true;
   }

test/functional/terminal/tui_spec.lua

@@ -836,6 +836,28 @@ describe('TUI background color', function()
       ..'", "-u", "NONE", "-i", "NONE", "--cmd", "set noswapfile"]')
   end)
 
+  it("triggers OptionSet event on terminal-response", function()
+    feed_data('\027:autocmd OptionSet background echo "did OptionSet, yay!"\n')
+
+    -- The child Nvim is running asynchronously; wait for it to register the
+    -- OptionSet handler.
+    feed_data('\027:autocmd OptionSet\n')
+    screen:expect({any='--- Autocommands ---'})
+
+    feed_data('\012')  -- CTRL-L: clear the screen
+    screen:expect([[
+      {1: }                                                 |
+      {4:~                                                 }|
+      {4:~                                                 }|
+      {4:~                                                 }|
+      {5:[No Name]                       0,0-1          All}|
+                                                        |
+      {3:-- TERMINAL --}                                    |
+    ]])
+    feed_data('\027]11;rgb:ffff/ffff/ffff\007')
+    screen:expect{any='did OptionSet, yay!'}
+  end)
+
   local function assert_bg(color, bg)
     it('handles '..color..' as '..bg, function()
       feed_data('\027]11;rgb:'..color..'\007:echo &background\n')