CSS outer box-shadow not clipped to border-box
According to the CSS specification, an outer box-shadow
should be clipped to the border-box of the the element:
An outer box-shadow casts a shadow as if the border-box of the element were opaque. Assuming a spread distance of zero, its perimeter has the exact same size and shape as the border box. The shadow is drawn outside the border edge only: it is clipped inside the border-box of the element.
Currently gnome-shell incorrectly renders the shadow "behind" the element, so that it remains visible if the element has a transparent background.
Some possible solutions to this were discussed in this thread.