Show/Hide Navigation Pane
This button toggles the Navigation Pane to either display or be hidden. Hiding the Navigation Pane makes better use of screen space on a smaller screens or when the main form is sized smaller. It also encourages focus on just the one screen.

(Hint: StoryCAD detects and reports spelling errors on most text fields.)
macOS
On macOS, the same toggle is available in the native menu bar under View > Toggle Navigation Pane.

By default the in-window toolbar is also visible. To hide it and rely on the menu bar instead, open Preferences and turn off Hide in-window toolbar on the Other tab. The change takes effect on the next launch.