In macOS 13.2, the NSTrackingEnabledDuringMouseDrag flag for NSTrackingArea is broken, i.e., mouse-entered and mouse-exited events are not sent during a drag. This problem did not exist in macOS 12. I've filed a bug report, FB11973492, just wondering if anyone knows a workaround.
I see now that there is a partial workaround. So long as the mouse is actually dragging data, as opposed to, say, dragging out a selection rectangle, one can register for dragged types and override draggingEntered: and draggingExited:
It amazes me that they can just break something like this. It's low level stuff that you'd think a lot of UIs depend on.