After further review, absolute max is 14 1/2 long side & 11 1/2 short side. Long side being where your keyboard plugs in."Reverse-stealth" builds do work out ok in cases that don't block boards over 9.6" wide--like this one. The FLP02 would be a beige option if it really takes 13" wide boards.
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Here's s shot of my Gigabyte X470 Aorus that I just installed and there is plenty of room on the board's perimeter.



