Restore a sensible buffer when replacing EXWM buffers

* exwm-layout.el (exwm-layout--other-buffer-exclude-exwm-mode-buffers)
(exwm-layout--other-buffer-exclude-buffers): New variables.
(exwm-layout--other-buffer-predicate): Allow excluding EXWM
buffers or buffers from a given set.
(exwm-layout--refresh): Replace EXWM buffers with sensible buffers
depending on the situation.  When in non-workspace/non-floating
frames, with some non-EXWM buffer; when the EXWM buffer is
displayed elsewhere, some buffer previously displayed in that
window (making sure it has been recently covered).

exwm-layout.el

@@ -240,6 +240,12 @@
       (xcb:flush exwm--connection)))
   (cl-incf exwm-layout--fullscreen-frame-count))
 
+(defvar exwm-layout--other-buffer-exclude-exwm-mode-buffers nil
+  "When non-nil, prevent EXWM buffers from being selected by `other-buffer'.")
+
+(defvar exwm-layout--other-buffer-exclude-buffers nil
+  "List of buffers that should not be selected by `other-buffer'.")
+
 (defun exwm-layout--other-buffer-predicate (buffer)
   "Return non-nil when the BUFFER may be displayed in selected frame.
 
@@ -247,9 +253,19 @@ Prevents EXWM-mode buffers already being displayed on some other window from
 being selected.
 
 Should be set as `buffer-predicate' frame parameter for all
-frames.  Used by `other-buffer'."
-  (not (and (eq 'exwm-mode (buffer-local-value 'major-mode buffer))
-            (get-buffer-window buffer t))))
+frames.  Used by `other-buffer'.
+
+When variable `exwm-layout--other-buffer-exclude-exwm-mode-buffers'
+is t EXWM buffers are never selected by `other-buffer'.
+
+When variable `exwm-layout--other-buffer-exclude-buffers' is a
+list of buffers, EXWM buffers belonging to that list are never
+selected by `other-buffer'."
+  (or (not (eq 'exwm-mode (buffer-local-value 'major-mode buffer)))
+      (and (not exwm-layout--other-buffer-exclude-exwm-mode-buffers)
+           (not (memq buffer exwm-layout--other-buffer-exclude-buffers))
+           ;; Do not select if already shown in some window.
+           (not (get-buffer-window buffer t)))))
 
 (defvar exwm-layout-show-all-buffers nil
   "Non-nil to allow switching to buffers on other workspaces.")
@@ -257,11 +273,8 @@ frames.  Used by `other-buffer'."
 (defun exwm-layout--refresh ()
   "Refresh layout."
   (let ((frame (selected-frame))
-        (placeholder (or (get-buffer "*scratch*")
-                         (progn
-                           (set-buffer-major-mode
-                            (get-buffer-create "*scratch*"))
-                           (get-buffer "*scratch*"))))
+        covered-buffers             ;EXWM-buffers covered by a new X window.
+        vacated-windows             ;Windows previously displaying EXWM-buffers.
         windows)
     (if (not (memq frame exwm-workspace--list))
         (if (frame-parameter frame 'exwm-outer-id)
@@ -276,10 +289,11 @@ frames.  Used by `other-buffer'."
           ;; Other frames (e.g. terminal/graphical frame of emacsclient)
           ;; We shall bury all `exwm-mode' buffers in this case
           (setq windows (window-list frame 0)) ;exclude minibuffer
+          (let ((exwm-layout--other-buffer-exclude-exwm-mode-buffers t))
             (dolist (window windows)
               (with-current-buffer (window-buffer window)
                 (when (eq major-mode 'exwm-mode)
-                (set-window-buffer window placeholder)))))
+                  (switch-to-prev-buffer window))))))
       ;; Refresh the whole workspace
       ;; Workspaces other than the active one can also be refreshed (RandR)
       (exwm--log "Refresh workspace %s" frame)
@@ -298,16 +312,31 @@ frames.  Used by `other-buffer'."
                     (exwm-layout--show exwm--id window)
                   (exwm-workspace-move-window
                    (cl-position frame exwm-workspace--list) exwm--id))
-                ;; Make sure this buffer is not displayed elsewhere
-                (dolist (i (get-buffer-window-list (current-buffer) 0 t))
-                  (unless (eq i window)
-                    (set-window-buffer i placeholder))))))))
+                ;; Make sure this buffer is not displayed elsewhere.  Note down
+                ;; windows displaying an EXWM-buffer now displayed elsewhere; we
+                ;; need to display with some other buffer there.
+                (setq vacated-windows
+                      (append vacated-windows (cdr (get-buffer-window-list
+                                                    (current-buffer) 0 t))))
+                ;; Note down when an EXWM-buffer is being covered by this
+                ;; buffer; we don't want it to reappear in some vacated window.
+                (let ((prev-buffer (car-safe
+                                    (car-safe (window-prev-buffers window)))))
+                  (and
+                   prev-buffer
+                   (eq 'exwm-mode (buffer-local-value 'major-mode prev-buffer))
+                   (push prev-buffer covered-buffers))))))))
+      ;; Set some sensible buffer to vacated windows.
+      (let ((exwm-layout--other-buffer-exclude-buffers covered-buffers))
+        (dolist (window vacated-windows)
+          (switch-to-prev-buffer window)))
       ;; Make sure windows floating / on other workspaces are excluded
+      (let ((exwm-layout--other-buffer-exclude-exwm-mode-buffers t))
         (dolist (window (window-list frame 0))
           (with-current-buffer (window-buffer window)
             (when (and (eq major-mode 'exwm-mode)
                        (or exwm--floating-frame (not (eq frame exwm--frame))))
-            (set-window-buffer window placeholder))))
+              (switch-to-prev-buffer window)))))
       ;; Update _NET_CLIENT_LIST_STACKING
       (xcb:+request exwm--connection
           (make-instance 'xcb:ewmh:set-_NET_CLIENT_LIST_STACKING