Created attachment 9779
Doubled decoration on non active windows
Not sure if it is actually a bug, but it looks a lot like one to me...
Whatever the Theme I use (Within the default ones: Greybird, Adwaita, High Contrast, Numen...), appart Adwaita-dark (but this might be the darkness that prevents from seeing it), the windows that are not selected get there Minimize/Maximize/Close button decoration dedoubled... See attached picture.
Look like a dupe of bug #16009
*** This bug has been marked as a duplicate of bug 16009 ***