fix(api): handle win_split_ins failure properly

Problem: nvim_win_set_config does not handle failure in win_split_ins properly
yet, which can cause all sorts of issues. Also nvim_open_win and
nvim_win_set_config do not set the error message to the one from win_split_ins.

Solution: handle failure by undoing winframe_remove, like in win_splitmove.
Make sure autocommands from switching to the altwin fire within a valid window,
and ensure they don't screw things up. Set the error message to that of
win_split_ins, if any.

Also change a few other small things, including:

- adjust win_append to take a tabpage_T * argument, which is more consistent
  with win_remove (and also allows us to undo a call to win_remove).

- allow winframe_restore to restore window positions. Useful if `wp` was in a
  different tabpage, as a call to win_comp_pos (which only works for the current
  tabpage) after winframe_restore should no longer be needed.

  Though enter_tabpage calls win_comp_pos anyway, this has the advantage of
  ensuring w_winrow/col remains accurate even before entering the tabpage
  (useful for stuff like win_screenpos, if used on a window in another tabpage).

  (This change should probably also be PR'd to Vim later, even though it doesn't
  use winframe_restore for a `wp` in a different tabpage yet).

1 files changed, 1 insertions, 1 deletions

diff --git a/src/nvim/autocmd.c b/src/nvim/autocmd.c
index 3f93906942..652b6ba74e 100644
--- a/src/nvim/autocmd.c
+++ b/src/nvim/autocmd.c
@@ -1333,7 +1333,7 @@ void aucmd_prepbuf(aco_save_T *aco, buf_T *buf)
 
     block_autocmds();  // We don't want BufEnter/WinEnter autocommands.
     if (need_append) {
-      win_append(lastwin, auc_win);
+      win_append(lastwin, auc_win, NULL);
       pmap_put(int)(&window_handles, auc_win->handle, auc_win);
       win_config_float(auc_win, auc_win->w_config);
     }