[FIX] Restore old code that disable popwin when Helm is active

Otherwise the window displayed by Helm i.e. help windows in by executing
persistent action in helm-M-x or helm-descbinds are messed up.

This change also preserves the fix introduced by syl20bnr#2081 for helm-resume.

spacemacs/packages.el

@@ -1510,13 +1510,43 @@ Removes the automatic guessing of the initial value based on thing at point. "
                                                      (inhibit-same-window . t)
                                                      (window-height . 0.4)))
       (defvar spacemacs-display-buffer-alist nil)
+      (defun spacemacs//helm-prepare-display ()
+        "Prepare necessary settings to make Helm display properly."
+        ;; avoid Helm buffer being diplaye twice when user
+        ;; sets this variable to some function that pop buffer to
+        ;; a window. See https://github.com/syl20bnr/spacemacs/issues/1396
+        (let ((display-buffer-base-action '(nil)))
+          (setq spacemacs-display-buffer-alist display-buffer-alist)
+          ;; the only buffer to display is Helm, nothing else we must set this
+          ;; otherwise Helm cannot reuse its own windows for copyinng/deleting
+          ;; etc... because of existing popwin buffers in the alist
+          (setq display-buffer-alist nil)
+          (popwin-mode -1)))
+
       (defun spacemacs//display-helm-at-bottom (buffer)
         (let ((display-buffer-alist (list spacemacs-helm-display-help-buffer-regexp
+                                          ;; this or any specialized case of Helm buffer must be added AFTER
+                                          ;; `spacemacs-helm-display-buffer-regexp'. Otherwise,
+                                          ;; `spacemacs-helm-display-buffer-regexp' will be used before
+                                          ;; `spacemacs-helm-display-help-buffer-regexp' and display
+                                          ;; configuration for normal Helm buffer is applied for helm help
+                                          ;; buffer, making the help buffer unable to be displayed.
                                           spacemacs-helm-display-buffer-regexp)))
           (helm-default-display-buffer buffer)))
 
       (setq helm-display-function 'spacemacs//display-helm-at-bottom)
 
+      (defun spacemacs//restore-previous-display-config ()
+        (popwin-mode 1)
+        ;; we must enable popwin-mode first then restore `display-buffer-alist'
+        ;; Otherwise, popwin keeps adding up its own buffers to `display-buffer-alist'
+        ;; and could slow down Emacs as the list grows
+        (setq display-buffer-alist spacemacs-display-buffer-alist))
+
+      (add-hook 'helm-after-initialize-hook 'spacemacs//helm-prepare-display)
+      ;;  Restore popwin-mode after a Helm session finishes.
+      (add-hook 'helm-cleanup-hook 'spacemacs//restore-previous-display-config)
+
       ;; Add minibuffer history with `helm-minibuffer-history'
       (define-key minibuffer-local-map (kbd "C-c C-l") 'helm-minibuffer-history)