Hi Chris,
This is a known issue with 1.8.5 on macOS, but it is a very minor issue, actually. Sometimes when you start dragging an item on macOS, the mouse cursor does not place itself outside the element you are dragging (it must be at the left side of the element). You know that this happens because either the mouse cursor does not change to indicate the drop action you're about to perform, or it changes incorrectly. When this happens, simply press Esc to abort the operation and try again, perhaps moving the mouse a bit more slowly. It works without problems. As you continue working with PixInsight, you'll notice that dragging actions become more responsive.
I am really sorry about these issues on macOS. The problem is
an unresolved Qt bug that has turned into a serious regression in the latest versions. I'll try to do my best to fix this problem as soon as possible.