Eyedropper won't select from outside PS CS5 on Mac running High Sierra

Hi

I've not used PS for quite sometime and, of course as luck would have it, having returned to it for a simple task it doesn't seem work. I want to use the Eyedropper to sample color in a window outside of PS.  I understand I ought to be able to left-click within my document and then drag the cursor outside the PS window to, for example, a webpage and sample the color.  As soon as the cursor moves outside of PS the sample color reverts to black.  I have reset the tool and reset my preferences.  Other ideas?

Thanks in advance

Steve