Comment 5 for bug 1521586

Can confirm this part

>The same thing happens with some gtk3 window shadows, e.g. the gnome calculator: Underlying
> windows cannot be brought into focus near the window border in the shadow area.

Ubuntu 18.04 gnome shell, multiple monitors, nvidia.

I first noticed as I had a window full screen on monitor 1, and it prevents the use of the "close,minimize,maximise" controls on the titlebar of neighbouring window on another monitor, unless I focus on it, by clicking outside the shadowed region.

I'm not 100% on who is at fault, but shadows of UI elements should be transparent to clicks, IMO.