Widget turns on/off with every button press in menu (LCD-Mode 'doublebuffered')

As the topic: in LCD-Mode 'doublebuffered' the widget bar at the top turns on and off, every time any button is pressed. Why the menu doesn't fill the whole screen? Is there a solution or workaround?

Minimum example:

Bangle.setLCDMode("doublebuffered");

Bangle.loadWidgets();

let menu = {
  "": { title: '--- DUMMY ---' },
  "Foo": 'Bar',
  "Exit": function() {
    E.showMenu();
  }
};

setWatch(function() {
  E.showMenu(menu);
}, BTN2, { repeat: true, edge: 'rising'});

Bangle.drawWidgets();
g.clear();
g.flip();
E.showMenu(menu);