Use `buffer-predicate' frame parameter to prevent switching to visible EXWM buffers

* exwm-layout.el (exwm-layout--other-buffer-predicate): New
function to be set as `buffer-predicate' frame parameter.
* exwm-workspace.el (exwm-workspace--init): Use above function on
workspace frames.

exwm-layout.el

@@ -240,6 +240,17 @@
       (xcb:flush exwm--connection)))
   (cl-incf exwm-layout--fullscreen-frame-count))
 
+(defun exwm-layout--other-buffer-predicate (buffer)
+  "Return non-nil when the BUFFER may be displayed in selected frame.
+
+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))))
+
 (defvar exwm-layout-show-all-buffers nil
   "Non-nil to allow switching to buffers on other workspaces.")
 

exwm-workspace.el

@@ -235,6 +235,7 @@ The optional FORCE option is for internal use only."
 (declare-function exwm-layout--show "exwm-layout.el" (id &optional window))
 (declare-function exwm-layout--hide "exwm-layout.el" (id))
 (declare-function exwm-layout--refresh "exwm-layout.el")
+(declare-function exwm-layout--other-buffer-predicate "exwm-layout.el" (buffer))
 
 ;;;###autoload
 (defun exwm-workspace-move-window (index &optional id)
@@ -684,6 +685,9 @@ The optional FORCE option is for internal use only."
     ;; The default behavior of `display-buffer' (indirectly called by
     ;; `minibuffer-completion-help') is not correct here.
     (cl-pushnew '(exwm-workspace--display-buffer) display-buffer-alist))
+  ;; Prevent `other-buffer' from selecting already displayed EXWM buffers.
+  (modify-all-frames-parameters
+   '((buffer-predicate . exwm-layout--other-buffer-predicate)))
   ;; Configure workspaces
   (dolist (i exwm-workspace--list)
     (let ((outer-id (string-to-number (frame-parameter i 'outer-window-id)))