This report is related (or: a subset of) bug #12006 Currently Xfwm4 shows shadows on maximized windows, which is - from my understanding and point of view - not necessary and if you have multiple displays, the shadow of a maximized window "bleeds over" to the next display, which simply looks weird. This minimal patch tells the compositor to ignore maximized windows when drawing shadows.
Created attachment 9383 Don't draw shadows on maximized windows
Just for completeness sake, this is one of the items that (despite the simple fix) I put on the 4.16 roadmap: https://wiki.xfce.org/releng/4.16/roadmap/xfwm4
Simon Steinbeiss referenced this bugreport in commit a74651cb53aa0f0cebfea1bd1fd2d361fdcd789f compositor: Don't draw shadow on maximized windows https://git.xfce.org/xfce/xfwm4/commit?id=a74651cb53aa0f0cebfea1bd1fd2d361fdcd789f
Simon Steinbeiss referenced this bugreport in commit d2b0e3cc7514ab1ed1a84044b47d6dd4865036c6 compositor: Don't draw shadow on maximized windows https://git.xfce.org/xfce/xfwm4/commit?id=d2b0e3cc7514ab1ed1a84044b47d6dd4865036c6