vim-patch:9.0.0789: dummy buffer ends up in a window

Problem:    Dummy buffer ends up in a window.
Solution:   Disallow navigating to a dummy buffer.
8f3c3c6cd0

src/nvim/buffer.c

@@ -1224,6 +1224,11 @@ int do_buffer(int action, int start, int dir, int count, int forceit)
     }
     return FAIL;
   }
+  if ((action == DOBUF_GOTO || action == DOBUF_SPLIT) && (buf->b_flags & BF_DUMMY)) {
+    // disallow navigating to the dummy buffer
+    semsg(_(e_nobufnr), count);
+    return FAIL;
+  }
 
   // delete buffer "buf" from memory and/or the list
   if (unload) {

src/nvim/testdir/test_autocmd.vim

@@ -3269,4 +3269,24 @@ func Test_noname_autocmd()
   augroup! test_noname_autocmd_group
 endfunc
 
+func Test_autocmd_split_dummy()
+  " Autocommand trying to split a window containing a dummy buffer.
+  auto BufReadPre * exe "sbuf " .. expand("<abuf>") 
+  " Avoid the "W11" prompt
+  au FileChangedShell * let v:fcs_choice = 'reload'
+  func Xautocmd_changelist()
+    cal writefile(['Xtestfile2:4:4'], 'Xerr')
+    edit Xerr
+    lex 'Xtestfile2:4:4'
+  endfunc
+  call Xautocmd_changelist()
+  call assert_fails('call Xautocmd_changelist()', 'E86:')
+
+  au! BufReadPre
+  au! FileChangedShell
+  delfunc Xautocmd_changelist
+  bwipe! Xerr
+  call delete('Xerr')
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab