Bangle.js 2: how to draw Widgets without clearing Widget area?

The Bangle.js 2 drawWidgets() code is here: https://github.com/espruino/Espruino/blob/master/libs/js/banglejs/Bangle_drawWidgets_Q3.js

So your app could override that with something like this:

Bangle.drawWidgets = function() {
  // Same as default Bangle.drawWidgets
  var w=g.getWidth(), h=g.getHeight();
  var pos = {
	  tl:{x:0, y:0, r:0, c:0}, // if r==1, we're right->left
	  tr:{x:w-1, y:0, r:1, c:0},
	  bl:{x:0, y:h-24, r:0, c:0},
	  br:{x:w-1, y:h-24, r:1, c:0}
  };
  if (global.WIDGETS) {
    for (var wd of WIDGETS) {
  	  var p = pos[wd.area];
  	  if (!p) continue;
  	  wd.x = p.x - p.r*wd.width;
  	  wd.y = p.y;
  	  p.x += wd.width*(1-2*p.r);
  	  p.c++;
    }
    g.reset();
    // Changes start here:
    if (pos.tl.c || pos.tr.c) {
      g.setClipRect(0,0,w-1,23);
     // TODO: redraw top widget area background
     g.reset();
   }
   if (pos.bl.c || pos.br.c) {
     g.setClipRect(0,h-24,w-1,h-1);
     // TODO: redraw bottom widget area background
     g.reset();
   }
    try {
        for (wd of WIDGETS) {
            // clear individual widget background:
            g.clearRect(wd.x,wd.y, wd.x+wd.width-1,23);
            wd.draw(wd); 
       }
   } catch(e) {print(e);}
  }
}

You want to override Bangle.drawWidgets() instead of just drawing them yourself, because widgets tend to call Bangle.drawWidgets() to show/hide themself.
And you need to redraw the background for the same reason: widgets can be shown/hidden, which moves other widgets around. (I guess you could keep track of widget positions to check if you really need to redraw)