This was discussed in Advent of Computing episode 150 "Starting Windows Up"[1,2] and the timeline of a 1983 demo which showed overlapping windows and multitasking, but also highlighted the contrast to the DR4 build from late 1984 claiming to introduce a multi-tasking scheduler.

This isn't really new information to the Stack Exchange question and answers, but it's kind of fun coincidental coverage of the topic.

[1] https://adventofcomputing.libsyn.com/episode-150-starting-wi...

[2] https://podscripts.co/podcasts/advent-of-computing/episode-1...