This extension for StarUML(http://staruml.io) adds two buttons in the statusbar of the interface (bottom bar) to toggle the Sidebar (left pane) and the Navigator (right pane) of the window.
You just have to install the extension, and restart or reload (CMD + R) StarUML 2. You will see two new buttons in the right section of the Statusbar.
You should know you can perform the same operation with the View > Hide Sidebarand View > Hide Navigator menus.