fix(misc): update setup_termbg_sync() to also handle transparent bg

Details:
- When a color scheme with transparent background is applied, terminal
  background sync did nothing. This could result into showing a
  different terminal bg color that was set by the previous sync.
  Instead of doing nothing - reset to the original terminal bg color.

Resolve #1955

Co-authored-by: Evgeni Chasnovski <evgeni.chasnovski@gmail.com>

Author: mintoya

lua/mini/misc.lua

@@ -319,26 +319,26 @@ MiniMisc.setup_termbg_sync = function()
     -- Neovim=0.10 uses string sequence as response, while Neovim>=0.11 sets it
     -- in `sequence` table field
     local seq = type(args.data) == 'table' and args.data.sequence or args.data
-    local ok, bg_init = pcall(H.parse_osc11, seq)
-    if not (ok and type(bg_init) == 'string') then return table.insert(bad_responses, seq) end
+    local ok, termbg = pcall(H.parse_osc11, seq)
+    if not (ok and type(termbg) == 'string') then return table.insert(bad_responses, seq) end
     had_proper_response = true
     pcall(vim.api.nvim_del_autocmd, track_au_id)
 
+    -- Set up reset to the color returned from the very first call
+    H.termbg_init = H.termbg_init or termbg
+    local reset = function() io.stdout:write('\027]11;' .. H.termbg_init .. '\007') end
+    vim.api.nvim_create_autocmd({ 'VimLeavePre', 'VimSuspend' }, { group = augroup, callback = reset })
+
     -- Set up sync
     local sync = function()
       local normal = vim.api.nvim_get_hl_by_name('Normal', true)
-      if normal.background == nil then return end
+      if normal.background == nil then return reset() end
       -- NOTE: use `io.stdout` instead of `io.write` to ensure correct target
       -- Otherwise after `io.output(file); file:close()` there is an error
       io.stdout:write(string.format('\027]11;#%06x\007', normal.background))
     end
     vim.api.nvim_create_autocmd({ 'VimResume', 'ColorScheme' }, { group = augroup, callback = sync })
 
-    -- Set up reset to the color returned from the very first call
-    H.termbg_init = H.termbg_init or bg_init
-    local reset = function() io.stdout:write('\027]11;' .. H.termbg_init .. '\007') end
-    vim.api.nvim_create_autocmd({ 'VimLeavePre', 'VimSuspend' }, { group = augroup, callback = reset })
-
     -- Sync immediately
     sync()
   end

tests/test_misc.lua

@@ -676,6 +676,21 @@ T['setup_termbg_sync()']['handles different color formats'] = function()
   validate('rgba:1/23/456/1234', '#112345')
 end
 
+T['setup_termbg_sync()']['handles transparent `Normal` background'] = function()
+  skip_if_no_010()
+
+  child.cmd('hi Normal guifg=#222222 guibg=#dddddd')
+  child.lua('MiniMisc.setup_termbg_sync()')
+  child.api.nvim_exec_autocmds('TermResponse', { data = '\27]11;rgb:1111/2626/2d2d' })
+  child.lua('_G.log = {}')
+
+  -- When syncing with "transparent" `Normal`, should restore the original
+  -- terminal background
+  child.cmd('hi Normal guifg=#222222 guibg=NONE')
+  child.api.nvim_exec_autocmds('ColorScheme', {})
+  eq(child.lua_get('_G.log'), { { '\27]11;#11262d\a' } })
+end
+
 local restore_cursor_test_file = make_path(dir_misc_path, 'restore-cursor.lua')
 local restore_cursor_init_file = make_path(dir_misc_path, 'init-restore-cursor.lua')
 local restore_cursor_shada_path = make_path(dir_misc_path, 'restore-cursor.shada')