vim-patch:9.0.1036: undo misbehaves when writing from an insert mode mapping

Problem:    Undo misbehaves when writing from an insert mode mapping.
Solution:   Sync undo when writing. (closes vim/vim#11674)

3f8f827723

Co-authored-by: Bram Moolenaar <Bram@vim.org>
(cherry picked from commit 70d6c335b1)

test/functional/editor/undo_spec.lua

@@ -9,6 +9,7 @@ local feed = helpers.feed
 local feed_command = helpers.feed_command
 local insert = helpers.insert
 local funcs = helpers.funcs
+local exec = helpers.exec
 
 local function lastmessage()
   local messages = funcs.split(funcs.execute('messages'), '\n')
@@ -67,6 +68,40 @@ describe('u CTRL-R g- g+', function()
     undo_and_redo(4, 'u', '<C-r>', '1')
     undo_and_redo(4, 'g-', 'g+', '1')
   end)
+
+  describe('undo works correctly when writing in Insert mode', function()
+    before_each(function()
+      exec([[
+        edit Xtestfile.txt
+        set undolevels=100 undofile
+        write
+      ]])
+    end)
+
+    after_each(function()
+      command('bwipe!')
+      os.remove('Xtestfile.txt')
+      os.remove('Xtestfile.txt.un~')
+    end)
+
+    -- oldtest: Test_undo_after_write()
+    it('using <Cmd> mapping', function()
+      command('imap . <Cmd>write<CR>')
+      feed('Otest.<CR>boo!!!<Esc>')
+      expect([[
+        test
+        boo!!!
+        ]])
+
+      feed('u')
+      expect([[
+        test
+        ]])
+
+      feed('u')
+      expect('')
+    end)
+  end)
 end)
 
 describe(':undo! command', function()