It's not actually glass, instead the apple engineers and designers are basically simulating effect of surface tension of drops of liquid. Unfortunately the refraction at the edges of a droplet is not informative about whether the droplet is inward or outward facing (i.e. if it it toggled on or off). Hence why they use additional highlights and shadow to indicate the 3D structure. The liquid effect is a total gimmick . And they added insult to injury by adding color-changes and movement which is totally distracting when you re scrolling that diffucult paper.