Merge pull request #22023 from neovim/fix-syntax

* fix(treesitter): properly restore `'syntax'` (#21358)
* fix(treesitter): really restore syntax
    - also unset b:ts_highlight on stop()
    Fixes: #21836

runtime/lua/vim/treesitter.lua

@@ -326,12 +326,8 @@ end
 ---@param lang (string|nil) Language of the parser (default: buffer filetype)
 function M.start(bufnr, lang)
   bufnr = bufnr or a.nvim_get_current_buf()
-
   local parser = M.get_parser(bufnr, lang)
-
   M.highlighter.new(parser)
-
-  vim.b[bufnr].ts_highlight = true
 end
 
 --- Stops treesitter highlighting for a buffer
@@ -343,8 +339,6 @@ function M.stop(bufnr)
   if M.highlighter.active[bufnr] then
     M.highlighter.active[bufnr]:destroy()
   end
-
-  vim.bo[bufnr].syntax = 'on'
 end
 
 return M

runtime/lua/vim/treesitter/highlighter.lua

@@ -89,6 +89,7 @@ function TSHighlighter.new(tree, opts)
   end
 
   vim.bo[self.bufnr].syntax = ''
+  vim.b[self.bufnr].ts_highlight = true
 
   TSHighlighter.active[self.bufnr] = self
 
@@ -114,6 +115,14 @@ function TSHighlighter:destroy()
   if TSHighlighter.active[self.bufnr] then
     TSHighlighter.active[self.bufnr] = nil
   end
+
+  if vim.api.nvim_buf_is_loaded(self.bufnr) then
+    vim.bo[self.bufnr].spelloptions = self.orig_spelloptions
+    vim.b[self.bufnr].ts_highlight = nil
+    if vim.g.syntax_on == 1 then
+      a.nvim_exec_autocmds('FileType', { group = 'syntaxset', buffer = self.bufnr })
+    end
+  end
 end
 
 ---@private