bug#32777: 27.0.50; window-buffer gets wrong point

[Federico Tedin <federicotedin@gmail.com>]

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> It seems like the problem could be solved by modifying the code on
> simple.el, line 1713 onward. I'll check and see if I can fix it.

I've managed to fix the problem, I'm attaching a patch with my proposed changes.

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From 6cb506a1f021888b5faf78090a4ab524fd8bb526 Mon Sep 17 00:00:00 2001
From: Federico Tedin <federicotedin@gmail.com>
Date: Mon, 1 Oct 2018 23:03:36 -0300
Subject: [PATCH] Fix M-n command completion for read-extended-command

* lisp/simple.el (read-extended-command): Ensure the function uses the
  apropiate window when looking for a command to suggest when user
  enters M-n after initial prompt. (Bug#32777)
---
 lisp/simple.el | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/lisp/simple.el b/lisp/simple.el
index e41630d4ed..1d129c3c99 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -1710,9 +1710,10 @@ read-extended-command
 	     (lambda ()
 	       ;; Get a command name at point in the original buffer
 	       ;; to propose it after M-n.
-	       (with-current-buffer (window-buffer (minibuffer-selected-window))
-		 (and (commandp (function-called-at-point))
-		      (format "%S" (function-called-at-point)))))))
+	       (with-selected-window (minibuffer-selected-window)
+                 (with-current-buffer (window-buffer (selected-window))
+		   (and (commandp (function-called-at-point))
+		        (format "%S" (function-called-at-point))))))))
     ;; Read a string, completing from and restricting to the set of
     ;; all defined commands.  Don't provide any initial input.
     ;; Save the command read on the extended-command history list.
-- 
2.17.1