When working with the Structure Board you can turn on Full Screen mode to give more screen space to your data. Full Screen mode can be toggled using the Toggle Panels menu or by pressing Z on your keyboard.
In Full Screen mode the JIRA application header is hidden and the main Structure toolbar becomes more compact. You can collapse the main toolbar by clicking the Collapse button to save even more screen space.
Bring the mouse cursor over the collapsed toolbar and the toolbar will show up temporarily, allowing you to perform a quick action.