patch 8.2.4166: undo synced when switching buffer in another window

Problem:    Undo synced when switching buffer in another window.
Solution:   Do not sync undo when not needed. (closes #9575)
diff --git a/src/testdir/test_timers.vim b/src/testdir/test_timers.vim
index 790b7b0..84fe05e 100644
--- a/src/testdir/test_timers.vim
+++ b/src/testdir/test_timers.vim
@@ -484,5 +484,28 @@
   call delete('XTest_timermessage')
 endfunc
 
+func Test_timer_using_win_execute_undo_sync()
+  let bufnr1 = bufnr()
+  new
+  let g:bufnr2 = bufnr()
+  let g:winid = win_getid()
+  exe "buffer " .. bufnr1
+  wincmd w
+  call setline(1, ['test'])
+  autocmd InsertEnter * call timer_start(100, { -> win_execute(g:winid, 'buffer ' .. g:bufnr2) })
+  call timer_start(200, { -> feedkeys("\<CR>bbbb\<Esc>") })
+  call feedkeys("Oaaaa", 'x!t')
+  " will hang here until the second timer fires
+  call assert_equal(['aaaa', 'bbbb', 'test'], getline(1, '$'))
+  undo
+  call assert_equal(['test'], getline(1, '$'))
+
+  bwipe!
+  bwipe!
+  unlet g:winid
+  unlet g:bufnr2
+  au! InsertEnter
+endfunc
+
 
 " vim: shiftwidth=2 sts=2 expandtab