override val root = gridpane() {
    minHeight = windowHeight
    minWidth = windowWidht
    prefHeight = windowHeight
    prefWidth = windowWidht
}

// java

static Gridpane gridpane(Consumer<Gridpane> action) {
    Gridpane gp = new Gridpane();
    action(gp);
    return gp;
}

Gridpane root = gridpane(gp -> {
    gp.minHeight = windowHeight
    gp.minWidth = windowWidht
    gp.prefHeight = windowHeight
    gp.prefWidth = windowWidht
});