fix: improve chat buffer rendering and state handling

- Make chat buffer non-modifiable by default and only allow modifications
  during content updates and after response is finished
- Join response progress undos together so user can undo whole ask/response at once
- Rerender headers in chat window on text change automatically, which means
  that when users accidentally edits the separators in buffer the highlights do not
  break until next question is asked

lua/CopilotChat/chat.lua

@@ -52,6 +52,16 @@ local Chat = class(function(self, help, on_buf_create)
     vim.bo[bufnr].filetype = self.name
     vim.bo[bufnr].syntax = 'markdown'
     vim.bo[bufnr].textwidth = 0
+    vim.bo[bufnr].modifiable = false
+
+    vim.api.nvim_create_autocmd({ 'TextChanged', 'TextChangedI' }, {
+      buffer = bufnr,
+      callback = function()
+        vim.defer_fn(function()
+          self:render()
+        end, 100)
+      end,
+    })
 
     if not self.spinner then
       self.spinner = Spinner(bufnr)
@@ -118,6 +128,7 @@ end
 
 function Chat:append(str)
   self:validate()
+  vim.bo[self.bufnr].modifiable = true
 
   if self:active() then
     utils.return_to_normal_mode()
@@ -145,17 +156,19 @@ function Chat:append(str)
     last_column,
     vim.split(str, '\n')
   )
-  self:render()
 
   if should_follow_cursor then
     self:follow()
   end
+
+  vim.bo[self.bufnr].modifiable = false
 end
 
 function Chat:clear()
   self:validate()
+  vim.bo[self.bufnr].modifiable = true
   vim.api.nvim_buf_set_lines(self.bufnr, 0, -1, false, {})
-  self:render()
+  vim.bo[self.bufnr].modifiable = false
 end
 
 function Chat:open(config)
@@ -233,7 +246,6 @@ function Chat:open(config)
   else
     vim.wo[self.winnr].foldcolumn = '0'
   end
-  self:render()
 end
 
 function Chat:close(bufnr)
@@ -295,6 +307,7 @@ function Chat:finish(msg, offset)
     msg = self.help
   end
 
+  vim.bo[self.bufnr].modifiable = true
   self:show_help(msg, -offset)
   if self.auto_insert and self:active() then
     vim.cmd('startinsert')

lua/CopilotChat/init.lua

@@ -740,6 +740,7 @@ function M.ask(prompt, config)
         on_progress = function(token)
           vim.schedule(function()
             if not config.no_chat then
+              vim.cmd('undojoin')
               state.chat:append(token)
             end
 

lua/CopilotChat/overlay.lua

@@ -18,6 +18,7 @@ local Overlay = class(function(self, name, help, on_buf_create)
   self.buf_create = function()
     local bufnr = vim.api.nvim_create_buf(false, true)
     vim.bo[bufnr].filetype = name
+    vim.bo[bufnr].modifiable = false
     vim.api.nvim_buf_set_name(bufnr, name)
     return bufnr
   end