Happens if either delta != 0 or the target not located in the top-left corner of the scene. Test below fails:

    public void testStandalone() {
        Button button = new Button("just a button nothing fancy");
        // location (relative to center of node) to fire at
        double deltaX = 20;
        double deltaY = 5;
        button.setOnMousePressed(mouse -> {
            // local coordinates
            double centerX = button.getWidth() / 2;
            double centerY = button.getHeight() / 2;
            assertEquals("local X", centerX + deltaX, mouse.getX(), 0.5);
            assertEquals("local Y", centerY + deltaY, mouse.getY(), 0.5);
            
            Point2D sceneP = button.localToScene(mouse.getX(), mouse.getY());
            assertEquals("scene X", sceneP.getX(), mouse.getSceneX(), 0.5);
            assertEquals("scene Y", sceneP.getY(), mouse.getSceneY(), 0.5);
            
            Point2D screenP = button.localToScreen(mouse.getX(), mouse.getY());
            assertEquals("screen X", screenP.getX(), mouse.getScreenX(), 0.5);
            assertEquals("screen Y", screenP.getY(), mouse.getScreenY(), 0.5);
        });
        MouseEventFirer firer = new MouseEventFirer(button);
        firer.fireMousePressed(deltaX, deltaY);
    }

Bubbled up while working on JDK-8253597 (trying to fire events into the indentation region of a treeTable leaf). The fix of that one will include a temporary hot fix (alternative event creation path) for this. Still needs a deeper look, though.