WinNT (maybe Win32): Mouse position during drag not consistent

When dragging a place window around in HotJava, at various points in time the
window will suddenly move to some weird (not random, but not easily characterizable) other place on the screen, and then move back. These points in time appear to be correlated to the mouse location crossing a component or a frame boundary.

To reproduce:

Bring up HotJava.

Go to some arbitrary (but valid) web page.

Select Places->Remember Place menu item.
Select Places->Show Remembered Places menu item. New Frame appears.

Select "New Places" in the top window.

Select your remembered place in the bottom window, and drag it around. Notice how it jerks around when you move it to certain places on the screen. The movement is being accomplished by getting the mouse coordinates as the drag proceeds and moving the drag window accordingly. Apparently, under some circumstances, the coordinates are wrong; at least on Solaris, the "right" thing happens.