I should also point out that the stacking order of the windows matters.
If I click on the empathy window (top-right, which casts the shadow) then move to the top-left workspace I see the bug. If I then click on the Chrome window (which already has focus!) the shadow goes away. It won't come back unless I click on the Empathy window (even if I switch back to that workspace and Empathy has focus).
I should also point out that the stacking order of the windows matters.
If I click on the empathy window (top-right, which casts the shadow) then move to the top-left workspace I see the bug. If I then click on the Chrome window (which already has focus!) the shadow goes away. It won't come back unless I click on the Empathy window (even if I switch back to that workspace and Empathy has focus).