vim-patch:partial:9.0.0077: wrong restored cursor position when switching window in autocmd

Problem:    When switching window in autocmd the restored cursor position may
            be wrong.
Solution:   Do not restore the cursor if it was not set. (closes vim/vim#10775)
b03950fafa

This patch cannot be fully ported because it depends on patch 8.2.3518.

src/nvim/testdir/test_autocmd.vim

@@ -2186,9 +2186,30 @@ func Test_autocmd_nested_cursor_invalid()
     au!
   augroup END
   set laststatus&
+  cclose
   bwipe!
 endfunc
 
+func Test_autocmd_nested_switch_window()
+  " run this in a separate Vim so that SafeState works
+  CheckRunVimInTerminal
+
+  let lines =<< trim END
+      vim9script
+      ['()']->writefile('Xautofile')
+      autocmd VimEnter * ++nested edit Xautofile | split
+      autocmd BufReadPost * autocmd SafeState * ++once foldclosed('.')
+      autocmd WinEnter * matchadd('ErrorMsg', 'pat')
+  END
+  call writefile(lines, 'Xautoscript')
+  let buf = RunVimInTerminal('-S Xautoscript', {'rows': 10})
+  call VerifyScreenDump(buf, 'Test_autocmd_nested_switch', {})
+
+  call StopVimInTerminal(buf)
+  call delete('Xautofile')
+  call delete('Xautoscript')
+endfunc
+
 func Test_autocmd_once()
   " Without ++once WinNew triggers twice
   let g:did_split = 0

src/nvim/window.c

@@ -6901,11 +6901,14 @@ void reset_lnums(void)
 {
   FOR_ALL_TAB_WINDOWS(tp, wp) {
     if (wp->w_buffer == curbuf) {
-      // Restore the value if the autocommand didn't change it.
+      // Restore the value if the autocommand didn't change it and it was
-      if (equalpos(wp->w_save_cursor.w_cursor_corr, wp->w_cursor)) {
+      // set.
+      if (equalpos(wp->w_save_cursor.w_cursor_corr, wp->w_cursor)
+          && wp->w_save_cursor.w_cursor_save.lnum != 0) {
         wp->w_cursor = wp->w_save_cursor.w_cursor_save;
       }
-      if (wp->w_save_cursor.w_topline_corr == wp->w_topline) {
+      if (wp->w_save_cursor.w_topline_corr == wp->w_topline
+          && wp->w_save_cursor.w_topline_save != 0) {
         wp->w_topline = wp->w_save_cursor.w_topline_save;
       }
     }