I am on windows.
In intelliJ Ultimate, there is at the top right corner of the window a horizontal tab labeled "1.Project". Pressing shift + esc will hide the file tree. However, pressing the same key combination does not unhide it.
How can I unhide this? Or create a custom binding that will toggle hide to unhide if it is already hidden?
Clément Baconnier was correct. alt + 1 was the key binding. cmd + 1 on osx.
Alternatively, I learned how to fish using the Action pane ctrl + shift + a that let me search for "Project" which exposed the keyboard shortcut.