Strange shadow-like behavior with GTK applications

[Matt Connell]

I am getting some strange window behavior with certain desktop applications.  Attached is a screenshot of evince, as an example of what I am seeing.

Is this a window shadow related behavior?  Is there a setting in the compositor that I need to tweak?  Is this related to the discussion on client-side decorations?

System info: Gentoo Linux, XFCE 4.14-r1 (from portage), using xfwm4, built-in compositor.
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