Tidy from code review comments.

Use 'dynamic-icon in leaf node to delegate icon rendering
functionality to parent node.

Remove tab action from leaf node.

Use arrow icons from lsp-treemacs theme and three spaces for leaf
nodes to align parent children correctly.

Remove commented code from expandable node and metals root definition.

Remove overriding return action for root, this does have the effect of
providing a treemacs error message and flash the element red - but
some people may prefer this.

Remove key definition for invoking treemacs node debug helper - not
required.

Fix file header with correct summary for treeview.

Use treemacs variables to define side window position and width.

When switching buffers ensure that the new buffer is associated with a
Metals server so that we don't attempt to switch treeviews on another
language server. In order to achieve this reliably we ensure that the
buffer being switched to has a major mode of scala-mode.

Author: dsyzling

lsp-metals-treeview.el

@@ -1,4 +1,4 @@
-;;; lsp-metals-treeview.el --- Scala Client settings             -*- lexical-binding: t; -*-
+;;; lsp-metals-treeview.el --- LSP Scala Metals Treeview   -*- lexical-binding: t; -*-
 
 ;; Copyright (C) 2019 Darren Syzling <[email protected]>
 
@@ -126,20 +126,28 @@ lsp-mode.")
   "When the treeview is displayed and visible this variable
 will hold the workspace associated with the instance.")
 
+(defun lsp--metals-treeview-position (slot)
+  "Side window position of Metals treeview with the given SLOT.
+Uses defaults for treemacs position and width."
+  `((side . ,treemacs-position)
+    (slot . ,slot)
+    (window-width . ,treemacs-width)))
+
 (defun lsp--metals-treeview-buffer-changed ()
   "When the buffer is switched check to see if a treeview
 is currently being displayed and whether we need to show
 an alternative workspace's treeview."
   (with-current-buffer (current-buffer)
-    (-if-let (workspaces (lsp-workspaces))
-        (progn
-          (when (and lsp--metals-treeview-active-view-workspace
-                     (not (member lsp--metals-treeview-active-view-workspace
-                                  workspaces)))
-            ;; hide current treeview and show new window associated with
-            ;; the current workspace of file in buffer.
-            (lsp--metals-treeview-hide-window lsp--metals-treeview-active-view-workspace)
-            (lsp--metals-treeview-show-window (car workspaces)))))))
+    (when (and (eq major-mode 'scala-mode)
+               (lsp-find-workspace lsp--metals-treeview-metals-server-id nil)
+               lsp--metals-treeview-active-view-workspace
+               (not (member lsp--metals-treeview-active-view-workspace
+                            (lsp-workspaces))))
+      
+      ;; hide current treeview and show new window associated with
+      ;; the current workspace of file in buffer.
+      (lsp--metals-treeview-hide-window lsp--metals-treeview-active-view-workspace)
+      (lsp--metals-treeview-show-window (car (lsp-workspaces))))))
 
 (defun lsp--metals-treeview-buffer-list-update ()
   (run-with-idle-timer lsp-metals-treeview-workspace-switch-delay
@@ -418,7 +426,7 @@ side window based based on an increasing SLOT number position."
   (when-let ((view (car views)))
     (lsp--metals-show-view workspace
                            (alist-get :view-id view)
-                           `((side . left) (slot . ,slot)))
+                           (lsp--metals-treeview-position slot))
     (lsp--metals-send-treeview-visibility-did-change workspace (alist-get :view-id view) t)
     (lsp--metals-display-views workspace (cdr views) (+ 1 slot))))
 
@@ -454,7 +462,7 @@ relative to the others. "
         (add-hook 'lsp-after-uninitialized-hook #'lsp--metals-treeview-on-workspace-shutdown))
 
     ;; No views are available - show temp message.
-    (lsp--metals-show-waiting-message workspace `((side . left) (slot . ,slot)))))
+    (lsp--metals-show-waiting-message workspace (lsp--metals-treeview-position slot))))
 
 (defun lsp--metals-treeview-refresh (workspace params)
   "Top level treeview changed - Metals has potentially given
@@ -645,11 +653,13 @@ expandable node. If the node isn't expandable for now do not show an icon. "
   (-if-let (icon (ht-get metals-node "icon"))
       (treemacs-get-icon-value icon nil "Metals")
     (if (ht-get metals-node "collapseState")
-        (if open-form?
-            (treemacs-as-icon "- " 'face 'font-lock-string-face)
-          (treemacs-as-icon "+ " 'face 'font-lock-string-face))
+        (treemacs-get-icon-value
+         (if open-form? 'expanded 'collapsed)
+         nil
+         lsp-treemacs-theme)
+        
       ;; leaf node without an icon
-      (treemacs-as-icon " " 'face 'font-lock-string-face))))
+      (treemacs-as-icon "   " 'face 'font-lock-string-face))))
 
 ;; to support not showing icons at all - leave for debugging for now
 ;; (defun lsp--metals-treeview-without-icons (metals-node)
@@ -722,11 +732,9 @@ collapsed or expanded."
 ;; We can possibly remove the leaf node definition and
 ;; replace lsp--metals-treeview-state to return treemacs-metals-node-closed-state
 ;;
-(treemacs-define-leaf-node metals-leaf
-  (treemacs-get-icon-value 'root nil "Metals")
+(treemacs-define-leaf-node metals-leaf 'dynamic-icon
 
   :ret-action #'lsp--metals-treeview-exec-node-action
-  :tab-action #'lsp--metals-treeview-exec-node-action
   :mouse1-action (lambda (&rest args)
                    (interactive)
                    (lsp--metals-treeview-exec-node-action args)))
@@ -742,9 +750,6 @@ collapsed or expanded."
 ;;
 
 (treemacs-define-expandable-node metals-node
-  ;; :icon-open (treemacs-as-icon "- " 'face 'font-lock-string-face)
-  ;; :icon-closed (treemacs-as-icon "+ " 'face 'font-lock-string-face)
-  
   :icon-open-form (lsp--metals-treeview-icon
                    (treemacs-button-get (treemacs-node-at-point) :node) t)
   :icon-closed-form (lsp--metals-treeview-icon
@@ -777,15 +782,10 @@ collapsed or expanded."
 ;;
 
 (treemacs-define-expandable-node metals-root
-  ;; :icon-open (treemacs-as-icon "- " 'face 'font-lock-string-face)
-  ;; :icon-closed (treemacs-as-icon "+ " 'face 'font-lock-string-face)
   :icon-open (treemacs-get-icon-value 'root nil "Metals")
   :icon-closed (treemacs-get-icon-value 'root nil "Metals")
   :query-function (lsp--metals-treeview-get-children lsp--metals-view-id)
 
-  ;; Ignore return action on root.
-  :ret-action '(lambda(&rest _))
-  
   :render-action
   (treemacs-render-node
    :icon (lsp--metals-treeview-icon item nil)
@@ -864,8 +864,6 @@ Metals Item: %s"
      (-some-> node (treemacs-button-get :parent) (treemacs--get-label-of))
      (treemacs-button-get node :eldoc)
      (-some-> node (treemacs-button-get :node)))))
-(global-set-key (kbd "C-x C-ö") #'treemacs-mu4e-debug-node)
-
 
 
 (provide 'lsp-metals-treeview)