Perhaps it is a part of a known issue but I don't know for sure.
Run the attached test on HiDPI Windows or setting -Dsun.java2d.uiScale.
The test sets a custom drag threshold and verifies that drag started with threshold+1 pixels movement.

Now, the test pass with an integer value of scale. Mouse pointer moves strictly in one direction (i.e. only right or left, as programmed). But with -Dsun.java2d.uiScale=2.5
(which is recommended scale on some HiDPI Windows 10 systems) it always fails, and the pointer movement is not direct: it moves first slightly vertically and only then horizontally.

Looks like there's a rounding problem somewhere.