This is something we are still investigating, but it seems like the drag-and-drop Flex call is completely broken in Adobe Air on macOS Sierra. We aren't going to wait around for Adobe or Apple, but we also aren't sure how much of this we will be able to workaround considering one of the major functions of the Flex codebase is not working.
Thankfully, this only affects reordering mockups and dragging stuff from the UI library (or outside the app) onto the canvas. The ladder issue can be mitigated by using quick add and the asset manager, it's the former issue that doesn't have an easy solution, and that's what we are focusing on.
We are going to work real hard to have something in folk's hands by Sierra's launch, so hang tight. We will keep you in the loop about this as the launch of Sierra gets closer.