With the Fantasy Grounds Unity update 2021-12-15, the Sidebar underwent a transformative and functional update.
Users used to have to choose which buttons or groups of buttons they needed for their game. This system as been replaced with a new collapsible group system that will be discussed below.
There are no longer settings in Options for the Sidebar.

Another update, v4.1.14 on 2022-03-03, added the vertically-collapsible Sidebar toggle allowing for three different configurations. This feature was added after the Collapsible Sidebar video tutorial at the bottom of the article.

Note: The following Sidebar example is what is shown when using D&D 5E ruleset and modules. Sidebar buttons will vary depending on which ruleset is loaded. For example, CoreRPG may have a Vehicles button specifically for vehicles, Basic Roleplaying (BRP) ruleset may have Powers, Professions, and Skills listed under the Character category, etc.

Adjustable Vertical Size Toggle

The Sidebar now has a an arrow icon at the bottom that allows it to be toggled between it’s three stages:

In all modes, Categories are still collapsible and expandible to allow for even further customization.

Collapsible Categories

The Sidebar’s collapsible categories make it easy to conserve screen space while still having all button options readily available at the click of a chevron. Each button now has an unobtrusive graphical representation for at-a-glance recognition.

Default

Double-Wide

Compressed

The sidebar, its buttons, frames, icons and text are all able to be themed by ruleset developers to match their ruleset aesthetic.

Sidebar Categories

The Sidebar is now broken down into the following categories:

Tool

Library

Player

Campaign

Character

Video Tutorial

https://www.youtube.com/watch?v=5FWuCBnd66E