emacs-31 73d959233e2: Fix setting windows' old_buffer in 'set-window-configuration' (Bug#81589)

branch: emacs-31
commit 73d959233e264d29cd327ac259c82ca4bc6d4510
Author: Martin Rudalics <[email protected]>
Commit: Martin Rudalics <[email protected]>

    Fix setting windows' old_buffer in 'set-window-configuration' (Bug#81589)
    
    * src/window.c (struct save_window_data): New slot change_stamp.
    (Fset_window_configuration): Restore the old_buffer of a window
    from the saved old_buffer only if the change stamps of the frame
    and save_window_data are the same (Bug#81589).
    (Fcurrent_window_configuration): Store FRAME's current change
    stamp in save_window_data.
---
 src/window.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/src/window.c b/src/window.c
index e4b0e62f3a3..022fb6a0d9f 100644
--- a/src/window.c
+++ b/src/window.c
@@ -7598,7 +7598,8 @@ struct save_window_data
 
     /* All fields above are traced by the GC.
        After saved_windows, the fields are ignored by the GC.  */
-
+    /* The change stamp of the frame at the time of saving.  */
+    int change_stamp;
     /* We should be able to do without the following two.  */
     int frame_cols, frame_lines;
     /* These three should get eventually replaced by their pixel
@@ -7839,7 +7840,25 @@ the return value is nil.  Otherwise the value is t.  */)
 	  /* If we squirreled away the buffer, restore it now.  */
 	  if (BUFFERP (w->combination_limit))
 	    wset_buffer (w, w->combination_limit);
-	  wset_old_buffer (w, p->old_buffer);
+
+	  if (data->change_stamp == f->change_stamp)
+	    /* Restore W's old_buffer slot but only if the configuration
+	       was saved and restored in between two redisplay cycles,
+	       that is, if F's saved change stamp and its current change
+	       stamp are equal.  In that case we should run W's buffer
+	       change functions provided the saved old_buffer and the
+	       restored buffer differ.  If the saved old_buffer and the
+	       restored buffer are one and the same, the window
+	       excursion was only temporary and it would be distracting
+	       to run the buffer change functions for it.
+
+	       If the change stamps are not equal, run the buffer change
+	       functions provided W's current buffer (which was stored
+	       by delete_all_child_windows above in W's old_buffer slot)
+	       and the buffer that will be restored differ (Bug#81079
+	       and Bug#81589).  */
+	    w->old_buffer = p->old_buffer;
+
 	  w->pixel_left = XFIXNAT (p->pixel_left);
 	  w->pixel_top = XFIXNAT (p->pixel_top);
 	  w->pixel_width = XFIXNAT (p->pixel_width);
@@ -8380,6 +8399,7 @@ saved by this function.  */)
   data->minibuf_selected_window = minibuf_level > 0 ? minibuf_selected_window : Qnil;
   data->root_window = FRAME_ROOT_WINDOW (f);
   data->focus_frame = FRAME_FOCUS_FRAME (f);
+  data->change_stamp = f->change_stamp;
   Lisp_Object tem = make_nil_vector (n_windows);
   data->saved_windows = tem;
   for (ptrdiff_t i = 0; i < n_windows; i++)