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.

1 files changed, 22 insertions, 0 deletions

diff --git a/test/functional/terminal/tui_spec.lua b/test/functional/terminal/tui_spec.lua
index 70356b7e2d..a0adb45630 100644
--- a/test/functional/terminal/tui_spec.lua
+++ b/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')