Comment 3 for bug 1521586

So, its not the shadow actually, which would have been weird in the first place. But some apps (I would say gtk3, csd; eg. gnome-calculator, gnome-software) show a different behavior concerning window border grabbing (for resizing). Windows, which can be resized, change the cursor the discussed area next to the window border and can be resized. This behavior is probably also applied to windows, which can not be resized, like gnome-calculator or the indicator menus. Thus leading to the several pixel wide region near window borders, where mouse clicks are not interpreted as, well, clicks.

If the border grabbing margin is intentional (although it leads to an inconsistent behavior between different apps), then it should be disabled for windows, which are not resizeable.