Sidebar menus in RT5?

Former RT3 user (it’s been a while), now installing RT5.
I vaguely remember sidebar menus in RT3, but RT5 only has navigation elements arranged - very confusingly, to my brain - in a row along the top-right corner.
Is there a way to enable any other navigation UI in RT5?

The main-navigation is top-left. The navigation for the shown object is upper-right.

We tuned our menues via html/Elements/Tabs. Copy it from share to local and fit it to your needs.
You’ll find all navigation in this file.

We mainly tuned the menue for $request_path =~ m{^/Ticket/}. It is still upper right, but support our workflows. E.g. the Actions are direct accessible while The Basics, Custom Fields and other not in the menue any more because one can access them via headlines of the Ticket’s metadata.

Don’t know this is the “right” way to do but it works well in our environment.

I have to correct my answer to this because I did not look into the html/Elements/Tabs before writing this text. Excuse me @Adam_Thompson .

Fact is one has to use callbacks to change the menue’s layout.

See: How to customize menues in RT5