The following failure happened one time in a test build on one of our macOS systems:

CallbackTest > testCustomPopup() FAILED
    org.opentest4j.AssertionFailedError: Method onResized was not called
        at app//org.junit.jupiter.api.AssertionUtils.fail(AssertionUtils.java:38)
        at app//org.junit.jupiter.api.Assertions.fail(Assertions.java:138)
        at app//test.javafx.scene.web.CallbackTest$TestUI.checkCalled(CallbackTest.java:246)
        at app//test.javafx.scene.web.CallbackTest.checkCustomPopup(CallbackTest.java:119)
        at app//test.javafx.scene.web.CallbackTest.testCustomPopup(CallbackTest.java:114)