How do I enable drag and drop in High Sierra?

There seems to be a new "feature" which has recently disabled drag and drop, I've tried creating an alternate account that fails the same way. Tinkering with trackpad/accessibility settings doesn't help either. I check the "enable drag and drop" box and try all the menu options but no joy.

Initially I thought it might be a file permissions issue, since the two folders (Users and Library) I was trying to move/copy to an external drive were from another Mac that had a sick hard drive. But other files that I own fail the same way.

No error messages.

I can drag, but not drop.

Running 10.13.6 on an Early 2011 MacBook Pro. I've got a 1tb hard drive formatted APFS if that matters (shouldn't), but I do understand APFS was intended primarily for SSD's.