Github user tbouron commented on the issue:
https://github.com/apache/brooklyn-ui/pull/99
The box shadow is there to indicate a notion of hierarchy. The side panels
come on top of the canvas, not beside it, i.e. they don't push the canvas to
left or right.
Material design says that in this particular case, the panel should be
"elevated", same as a piece of paper on top of another one. This creates a
shadow, reflected by the box shadow there currently is.
Brooklyn UI was created with material design in mind. It is poorly executed
in a lot of places but still. I think it make sense to keep this notion of
hierarchy as it fit with what it's doing
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