I went and checked, and it was an SPE article from 1990[1]: “Why X is not our ideal window system” by Gajewska, Manasse, and McCormack. Section 3 has extensive diagrams of various races, including ones between clients and the window manager. Was even discussed here at one point[2], it turns out. Where I came across it I’m not sure (on X.org[3] possibly?).