I can't find a specific feature request that exists for this (there is a discussion thread here, but it is marked as "solved" with a link to a totally unrelated feature).
I often want to share a photo from my library in Slack, imessages, e-mail, etc., and the easiest workflow on Mac these days for most apps is to be able to copy (cmd-C) the image and paste it into the other app. Dragging the photo to the other app is a nice alternative (though one I use less often).
For desktop lightroom, the best solution seems to be one of:
1.
* export photo
* drag the photo from Finder to the app of your choice
* delete the photo when done
2.
* cmd-ctrl-4 (for cropped screengrab) and drag the rectangle to match the edges of the photo as best you can
* paste poorly compressed photo into app of your choice (I just did it on a photo covering less than half my screen, and it resulted in a 25.4 MB file)
Surely it wouldn't be that hard to integrate Lightroom into the copy-paste mechanics of the OS so that we could (a) cmd-C to copy a lightweight PNG export of the photo to clipboard and (b) drag from grid view to achieve the same thing?