When a user begins to drag a file in Explorer, mouse messages (and in particular, WM_MOUSEMOVE) are no longer sent to any Explorer windows, including the items-view child window. You can confirm this using Spy++.
So if you're subclassing the items-view child window, is there any other ideal way to obtain the mouse coordinates of the current drag operation? I don't see any shell object methods, and I don't think you can do it with UIAutomation, but perhaps I'm overlooking something. The only idea I have is to trap WM_NCHITTEST, which appears to keep getting sent (apparently on a timer) while the drag is active.