Hi Beams, sorry for the trouble. This definitely should work for you. I just tested it out, and it seems to work properly for me. As soon as I click and drag on the grabber, the mockup is selected, and a black line shows where the mockup would be.
One thing that I noticed when testing this out in List view (instead of Thumbnail view), is that it works better if I click one first to select the mockup, and then click and drag again to reorder the mockup. I don’t know if that will help you or not.
We’d love to try and reproduce this. I don’t know that we intentionally fixed anything in this area, but just to make sure, could you try updating to the latest beta build? They’re coming hot and fast!
If you still have trouble, could you try getting a screenshot, or a screen recording or something? I’d love to see what you are seeing and get this figured out. Thanks!