(Fminibuffer_selected_window): New function.

(syms_of_window): Defsubr it.

src/window.c

@@ -4549,6 +4549,20 @@ by this function.  */)
   return result;
 }
 
+DEFUN ("minibuffer-selected-window", Fminibuffer_selected_window, Sminibuffer_selected_window, 0, 0, 0,
+       doc: /* Return the window which was selected when entering the minibuffer.
+Returns nil, if current window is not a minibuffer window.  */)
+     ()
+{
+  if (minibuf_level > 0
+      && MINI_WINDOW_P (XWINDOW (selected_window))
+      && !NILP (minibuf_selected_window)
+      && WINDOW_LIVE_P (minibuf_selected_window))
+    return minibuf_selected_window;
+
+  return Qnil;
+}
+
 /* Value is the number of lines actually displayed in window W,
    as opposed to its height.  */
 
@@ -6012,6 +6026,7 @@ This variable automatically becomes buffer-local when set.  */);
   defsubr (&Sscroll_right);
   defsubr (&Sother_window_for_scrolling);
   defsubr (&Sscroll_other_window);
+  defsubr (&Sminibuffer_selected_window);
   defsubr (&Srecenter);
   defsubr (&Swindow_text_height);
   defsubr (&Smove_to_window_line);