[Interface] Create command to start treemacs on boot.

Fixes #1111

Changelog.org

@@ -6,6 +6,7 @@
 - Deprecated ~treemacs-window-background-color~ in favour of ~treemacs-window-background-face~ and
   ~treemacs-hl-line-face~
 - Added ~treemacs-copy-absolute-path-at-point~
+- Added ~treemacs-start-on-boot~
 - Made it possible to disable workspace with a ~COMMENT~ directive
 - Added option to sort alphabetic-numerically (as with ~string-version-lessp~)
 - Added ~:on-expand~ and ~:on-collapse~ options to treelib nodes

README.org

@@ -560,6 +560,8 @@ discoverability.
     :after (treemacs)
     :ensure t
     :config (treemacs-set-scope-type 'Tabs))
+
+  (treemacs-start-on-boot)
 #+END_SRC
 
 * Configuration

src/elisp/treemacs.el

@@ -171,6 +171,20 @@ visiting a file or Emacs cannot find any tags for the current file."
        (setq treemacs-window (selected-window)))
      (treemacs--do-follow-tag index treemacs-window buffer-file project))))
 
+;;;###autoload
+(defun treemacs-start-on-boot (&optional focus-treemacs)
+  "Initializer specifically to start treemacs as part of your init file.
+
+Ensures that all visual elements are present which might otherwise be missing
+because their setup requires an interactive command or a post-command hook.
+
+FOCUS-TREEMACS indicates whether the treemacs window should be selected."
+  (-let [initial-window (selected-window)]
+    (treemacs)
+    (hl-line-highlight)
+    (redisplay)
+    (unless focus-treemacs (select-window initial-window))))
+
 ;;;###autoload
 (defun treemacs-select-window (&optional arg)
   "Select the treemacs window if it is visible.