I have routinely used photos on my computer - MacBook Pro (with a connection to a Dell monitor giving me two screens I need for work) - but suddenly the other day - the regular Mojave desktop started showing on BOTH monitors (they both had been different - with the photos routinely showing for 1 minute - on the Dell desktop. I did some internet sleuthing but couldn't fine a reason that the area where I choose photos is greyed out. See screenshot.
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