fix(defaults): wait until VimEnter to set background (#26284)

The OptionSet autocommand does not fire until Vim has finished starting,
so setting 'background' before the VimEnter event would not fire the
OptionSet event. The prior implementation also waited until VimEnter to
set 'background', so this was a regression introduced when moving
background detection into Lua.

runtime/lua/vim/_defaults.lua

@@ -252,14 +252,33 @@ do
   if tty then
     local timer = assert(vim.uv.new_timer())
 
+    ---@param bg string New value of the 'background' option
+    local function setbg(bg)
+      if vim.api.nvim_get_option_info2('background', {}).was_set then
+        -- Don't do anything if 'background' is already set
+        return
+      end
+
+      -- Wait until Nvim is finished starting to set 'background' to ensure the
+      -- OptionSet event fires.
+      if vim.v.vim_did_enter == 1 then
+        if vim.o.background ~= bg then
+          vim.o.background = bg
+        end
+      else
+        vim.api.nvim_create_autocmd('VimEnter', {
+          once = true,
+          nested = true,
+          callback = function()
+            setbg(bg)
+          end,
+        })
+      end
+    end
+
     local id = vim.api.nvim_create_autocmd('TermResponse', {
       nested = true,
       callback = function(args)
-        if vim.api.nvim_get_option_info2('background', {}).was_set then
-          -- Don't do anything if 'background' is already set
-          return true
-        end
-
         local resp = args.data ---@type string
         local r, g, b = parseosc11(resp)
         if r and g and b then
@@ -270,9 +289,7 @@ do
           if rr and gg and bb then
             local luminance = (0.299 * rr) + (0.587 * gg) + (0.114 * bb)
             local bg = luminance < 0.5 and 'dark' or 'light'
-            if bg ~= vim.o.background then
-              vim.o.background = bg
-            end
+            setbg(bg)
           end
 
           return true