vim-patch:9.0.0910: setting lines in another buffer may not work well

Problem:    Setting lines in another buffer may not work well.
Solution:   Make sure the buffer being changed has a window. (issue vim/vim#11558)

vim/vim@c934bfa

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/eval/buffer.c

@@ -76,25 +76,35 @@ static void set_buffer_lines(buf_T *buf, linenr_T lnum_arg, bool append, typval_
 {
   linenr_T lnum = lnum_arg + (append ? 1 : 0);
   long added = 0;
-  buf_T *curbuf_save = NULL;
   win_T *curwin_save = NULL;
-  const bool is_curbuf = buf == curbuf;
+  aco_save_T aco;
+  int using_aco = false;
   const bool save_VIsual_active = VIsual_active;
 
   // When using the current buffer ml_mfp will be set if needed.  Useful when
   // setline() is used on startup.  For other buffers the buffer must be
   // loaded.
+  const bool is_curbuf = buf == curbuf;
   if (buf == NULL || (!is_curbuf && buf->b_ml.ml_mfp == NULL) || lnum < 1) {
     rettv->vval.v_number = 1;  // FAIL
     return;
   }
 
   if (!is_curbuf) {
+    // Set "curbuf" to the buffer being changed.  Then make sure there is a
+    // window for it to handle any side effects.
     VIsual_active = false;
-    curbuf_save = curbuf;
     curwin_save = curwin;
     curbuf = buf;
-    find_win_for_curbuf();
+    find_win_for_curbuf();  // simplest: find existing window for "buf"
+
+    if (curwin->w_buffer != buf) {
+      // No existing window for this buffer.  It is dangerous to have
+      // curwin->w_buffer differ from "curbuf", use the autocmd window.
+      curbuf = curwin->w_buffer;
+      aucmd_prepbuf(&aco, buf);
+      using_aco = true;
+    }
   }
 
   linenr_T append_lnum;
@@ -193,8 +203,12 @@ static void set_buffer_lines(buf_T *buf, linenr_T lnum_arg, bool append, typval_
 
 done:
   if (!is_curbuf) {
-    curbuf = curbuf_save;
-    curwin = curwin_save;
+    if (using_aco) {
+      aucmd_restbuf(&aco);
+    } else {
+      curwin = curwin_save;
+      curbuf = curwin->w_buffer;
+    }
     VIsual_active = save_VIsual_active;
   }
 }