How to show a selected entity in front with PostProcessStage

CesiumJS
1

I want add a hover effect like this ,
hover

I found a example from sandcastle https://sandcastle.cesium.com/?src=Custom%20Per-Feature%20Post%20Process.html&label=Post%20Processing.

My quetions:

  1. How to bring selected entity top most?
  2. how to make selected entity transparent?
    gl_FragColor = vec4(highlighted, 0); seams not work