X11R3
The October 1988 X release in which the Xt toolkit took the shape that Motif and Athena went on to build on.
R3 shipped on 25 October 1988, only months after R2 but with a great deal more in it. The big change was in the X Toolkit Intrinsics: the global state of the early versions gave way to an explicit application context, and the interface was frozen firmly enough that vendors dared to build their own widget sets on top. That was the foundation OSF would soon put Motif on.
The contrib tape had swollen considerably, carrying window managers, games and experiments, including early work on what became GWM. What was still missing were the rules for how clients and window managers should get along; ICCCM was circulating as a draft and was only finished in R4. In the meantime every window manager interpreted the situation for itself, which explains a good deal of the creative behaviour of the era.