screenChangeHook

var ScreenType = {
    start_screen: "start_screen",
    hud: "hud_screen",
    ingame: "in_game_play_screen",
    survival_inv: "survival_inventory_screen",
    creative_inv: "creative_inventory_screen",
    pause_screen: "pause_screen",
    options_screen: "options_screen"
};

var currentScreen;

function screenChangeHook(screenName) {
    ctx.runOnUiThread(new java.lang.Runnable({
        run: function() {
            currentScreen = screenName;
            if(screenName == ScreenType.start_screen) {
                //show your button now
            } else {
                if(yourButtonGUI != null) {
                    if(yourButtonGUI.isShowing()) {
                        //hide your button now
                    }
                }
            }
        }
    }));
}