Handle window buffer changing

Example bug: Edit. C-c C-c such that the REPL buffer replaces the edit
buffer in the same window. Or, invoke magit and the magit buffer
replaces the edit buffer in the same window.

See also commit 689d0a6

racket-pdb.el

@@ -350,9 +350,10 @@ for the annotations resulting from the user's later edits.")
           (run-with-idle-timer racket-pdb-after-change-refresh-delay
                                nil      ;no repeat
                                #'racket--pdb-on-change-idle-timer
-                               (selected-window)))))
+                               (selected-window)
+                               (current-buffer)))))
 
-(defun racket--pdb-on-change-idle-timer (window)
+(defun racket--pdb-on-change-idle-timer (window buffer)
   "Handle after-change-hook => idle-timer expiration.
 
 One scenario to keep in mind: The user has typed a few characters
@@ -367,12 +368,12 @@ and might miss a change from before they even started completion
 -- which is not great, but is better than making a mistake
 rescheduling an idle-timer with an amount <= the amount of idle
 time that has already elapsed: see #504."
-  (let ((buffer (window-buffer window)))
-    (when (buffer-live-p buffer)
-      (with-current-buffer buffer
-        (unless (racket--pdb-completing-p)
-          (with-selected-window window
-            (racket-pdb-analyze)))))))
+  (when (and (buffer-live-p buffer)
+             (equal buffer (window-buffer window)))
+    (with-current-buffer buffer
+      (unless (racket--pdb-completing-p)
+        (with-selected-window window
+          (racket-pdb-analyze))))))
 
 (defun racket--pdb-completing-p ()
   "Is completion underway?