To reproduce, run the system tests on a system running Windows 10 version 1803 with Hi-DPI scaling at 125% as follows:

$ gradle -PFULL_TEST=true -PUSE_ROBOT=true :systemTests:test --tests MouseLocationOnScreenTest

test.robot.javafx.scene.MouseLocationOnScreenTest > testMouseLocation FAILED
    java.lang.AssertionError: expected:<54> but was:<53>
        at org.junit.Assert.fail(Assert.java:91)
        at org.junit.Assert.failNotEquals(Assert.java:645)
        at org.junit.Assert.assertEquals(Assert.java:126)
        at org.junit.Assert.assertEquals(Assert.java:470)
        at org.junit.Assert.assertEquals(Assert.java:454)
        at test.robot.javafx.scene.MouseLocationOnScreenTest.validate(MouseLocationOnScreenTest.java:115)
        at test.robot.javafx.scene.MouseLocationOnScreenTest.edge(MouseLocationOnScreenTest.java:123)
        at test.robot.javafx.scene.MouseLocationOnScreenTest.lambda$testMouseLocation$1(MouseLocationOnScreenTest.java:83)

The exact point at which it fails will differ depending on where the mouse is on the screen when the tests starts running. The failure mode suggests some sort of rounding error or similar product bug in the mouse move code, which was modified to work on Windows 10 and now seems to have some slight inaccuracies with a later version.