aucmd_prepbuf: also restore `prevwin` #9741

bisected to f5d5da391715

Other test steps:

    nvim -u NORC
    :terminal tree /    " Produces lots of output
    :edit somefile.txt
    :vsplit
    :vsplit
    <c-w>l
    <c-w>l
    <c-w>h
    <c-w>p

3 files changed, 19 insertions, 15 deletions

diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index 48cef9b1e7..d35e3b49d8 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -987,18 +987,16 @@ typedef struct {
                                           .relative = 0, .external = false, \
                                           .focusable = true })
 
-/*
- * Structure which contains all information that belongs to a window
- *
- * All row numbers are relative to the start of the window, except w_winrow.
- */
+/// Structure which contains all information that belongs to a window.
+///
+/// All row numbers are relative to the start of the window, except w_winrow.
 struct window_S {
   handle_T handle;                  ///< unique identifier for the window
 
   buf_T       *w_buffer;            ///< buffer we are a window into (used
                                     ///< often, keep it the first item!)
 
-  synblock_T  *w_s;                 /* for :ownsyntax */
+  synblock_T  *w_s;                 ///< for :ownsyntax
 
   int w_hl_id_normal;               ///< 'winhighlight' normal id
   int w_hl_attr_normal;             ///< 'winhighlight' normal final attrs
@@ -1008,18 +1006,18 @@ struct window_S {
 
   int w_hl_needs_update;            ///< attrs need to be recalculated
 
-  win_T       *w_prev;              /* link to previous window */
-  win_T       *w_next;              /* link to next window */
-  bool w_closing;                   /* window is being closed, don't let
-                                       autocommands close it too. */
+  win_T       *w_prev;              ///< link to previous window
+  win_T       *w_next;              ///< link to next window
+  bool w_closing;                   ///< window is being closed, don't let
+                                    ///  autocommands close it too.
 
-  frame_T     *w_frame;             /* frame containing this window */
+  frame_T     *w_frame;             ///< frame containing this window
 
-  pos_T w_cursor;                   /* cursor position in buffer */
+  pos_T w_cursor;                   ///< cursor position in buffer
 
-  colnr_T w_curswant;               /* The column we'd like to be at.  This is
-                                       used to try to stay in the same column
-                                       for up/down cursor motions. */
+  colnr_T w_curswant;               ///< Column we want to be at.  This is
+                                    ///  used to try to stay in the same column
+                                    ///  for up/down cursor motions.
 
   int w_set_curswant;               // If set, then update w_curswant the next
                                     // time through cursupdate() to the
diff --git a/src/nvim/fileio.c b/src/nvim/fileio.c
index 7be4107c94..53b945d983 100644
--- a/src/nvim/fileio.c
+++ b/src/nvim/fileio.c
@@ -6527,6 +6527,7 @@ aucmd_prepbuf (
     win = curwin;
 
   aco->save_curwin = curwin;
+  aco->save_prevwin = prevwin;
   aco->save_curbuf = curbuf;
   if (win != NULL) {
     /* There is a window for "buf" in the current tab page, make it the
@@ -6624,6 +6625,8 @@ win_found:
       // Hmm, original window disappeared.  Just use the first one.
       curwin = firstwin;
     }
+    prevwin = win_valid(aco->save_prevwin) ? aco->save_prevwin
+              : firstwin;  // window disappeared?
     vars_clear(&aucmd_win->w_vars->dv_hashtab);      // free all w: variables
     hash_init(&aucmd_win->w_vars->dv_hashtab);       // re-use the hashtab
     curbuf = curwin->w_buffer;
@@ -6656,6 +6659,8 @@ win_found:
       }
 
       curwin = aco->save_curwin;
+      prevwin = win_valid(aco->save_prevwin) ? aco->save_prevwin
+                : firstwin;  // window disappeared?
       curbuf = curwin->w_buffer;
       // In case the autocommand moves the cursor to a position that does not
       // exist in curbuf
diff --git a/src/nvim/fileio.h b/src/nvim/fileio.h
index 8db4b89806..a6011ec414 100644
--- a/src/nvim/fileio.h
+++ b/src/nvim/fileio.h
@@ -23,6 +23,7 @@ typedef struct {
   buf_T       *save_curbuf;     ///< saved curbuf
   int use_aucmd_win;            ///< using aucmd_win
   win_T       *save_curwin;     ///< saved curwin
+  win_T       *save_prevwin;    ///< saved prevwin
   win_T       *new_curwin;      ///< new curwin
   bufref_T     new_curbuf;      ///< new curbuf
   char_u      *globaldir;       ///< saved value of globaldir