vim-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 vim/vim#9575)
e615db0604

src/nvim/buffer.c

@@ -1454,7 +1454,10 @@ void set_curbuf(buf_T *buf, int action)
     }
     if (bufref_valid(&prevbufref) && !aborting()) {
       win_T *previouswin = curwin;
-      if (prevbuf == curbuf) {
+      // Do not sync when in Insert mode and the buffer is open in
+      // another window, might be a timer doing something in another
+      // window.
+      if (prevbuf == curbuf && ((State & INSERT) == 0 || curbuf->b_nwindows <= 1)) {
         u_sync(false);
       }
       close_buffer(prevbuf == curwin->w_buffer ? curwin : NULL,

src/nvim/testdir/test_timers.vim

@@ -379,4 +379,27 @@ func Test_timer_invalid_callback()
   call assert_fails('call timer_start(0, "0")', 'E921')
 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