I noticed the blur only "sees" the underlying pixels directly below the glass surface. Any pixels outside that box, but within the blur radius do not get used in the gaussian filter. Probably the edge pixels are just repeated. You can see this when the light of the moon pops in to view when the edge of the rectangle starts to touch the moon. It would look more real if the light pixels from the moon start to through even when the box itself is still just over the dark area.

Would this be possible to achieve in CSS? I presume having a larger box with the blur, but clipping it to a smaller box or something like that.