Bangle.js 2 how to keep backlight off while running in an app

I'm making an outdoor app. Apparently, the backlight is not needed under the sunlight and it will save battery as well. I also want to use the touch screen without unlocking the watch while I'm using it. So, I have settings as shown below.

Bangle.setOptions({backlightTimeout: 0}); 
Bangle.setBacklight(0);
Bangle.setLocked(false);

It works fine, but when I call either E.showMessage() or E.showPrompt(), the backlight turns on. It seems these E classes overrides the backlight setting in the code. I can immediately call Bangle.setBacklight(0); to turn it off, but wonder if there is a way to keep the backlight off while I'm running the app.

Thanks.