Change the window layout in iMovie on Mac
Change the size of the browser, viewer, and timeline
Show or hide the Libraries list
In the iMovie app
on your Mac, click the Libraries List button 
at the top of the browser.
Show or hide the browser
In the iMovie app
on your Mac, open a project in the timeline, and then click the Content Library button 
in the toolbar.
Restore the default window layout
If you have made adjustments to the iMovie window layout, you can easily restore the original layout.
In the iMovie app
on your Mac, choose Window > Revert to Original Layout.