Please Reset the Lightroom Preferences back to default and it should take care this Issue.
In Mac OS 10.9.x and later, to reset Lightroom's preferences, after you delete the above file, restart your Macintosh. Also, beginning with Mac OS 10.9, to fully reset your Lightroom preferences, in addition to deleting the plist file, press the shift + option keys immediately after you restart Lightroom, and select Reset Preferences from the resulting dialog box.
To reset your preferences in Windows, restart Lightroom from the pinned application icon in the Start Menu (Windows 7) or the Start Screen (Windows 8 and later), and immediately press the shift + alt keys. In the resulting Reset Preferences dialog box, indicate you want to reset your preferences.
Also there's known for shortcut F in Lightroom for Mac users, please refer : Lightroom and El Capitan | Mac OS 10.11
Let us know if that helps.
Thank you. I fixed it. I read someone else's similar problem and followed what they did. CTL/right click in the border. There I saw the choices from white to different gray to black.
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