The BRIEF text editor was also launched in 1985, and it incorporated a tiling windows manager (implemented in the MS-DOS text video mode, and using the MS-DOS graphic characters for window decorations, menus etc.).
That was the first tiling windows manager that I have used, and it worked much better than many of the much more recent tiling windows managers.
I believe that the BRIEF text editor for programmers was used much more widely than Windows 1.0, which was mostly a toy.
The video mentions that the editor running in one of the windows is Emacs. Since the video is from 1988, this would be Emacs about version 18.50¹, which did, IIRC, have split-window functionality. So they had, in the video, another, running, (albeit text-only) implementation of tiling windows.
I believe that the BRIEF text editor was strongly inspired by EMACS (by the original EMACS, because the GNU Emacs was launched almost simultaneously with BRIEF).
Like from EMACS, you could do pretty much anything from inside the editor and BRIEF also had a very powerful LISP-like macro language, from which you could modify the behavior of any key press and so on.
So there is no wonder if its window system was also inspired by EMACS (assuming that the original EMACS already had tiling windows, which I do not know).
The MIT Lisp Machine had a version of Emacs written in Lisp called ZWEI and later called Zmacs.
The Lisp Machine window system had tiling features in its window manager and also a split screen feature in ZWEI. The 1980 sources of the Lisp operating system had that, but it might have been developed earlier.
And Xerox Star (1981) and CEDAR (1982) before that. (I have a vague memory of AppleWorks supporting tiled windows too, although application-level implementations probably don't count.)
Update: Huzzah! 1982, It was Rob Pike's "Blit", the Bell Labs Intelligent Terminal, pictured here tiling with a connection to SDF! I knew I wasn't losing my mind.
It seems that tiling was an obvious approach early on when figuring out clipping and partial redrawing got in the way of getting the product out the door. Windows 1 on an EGA screen just didn't leave a lot of space for the applications if there was more than one window.
Overlapping windows was really the only way to manage a small screen. I had a 19" 1152x882 screen in 1992 and people were always surprised to see it, as 13-14" 800x600 were typical at the time and lots of people were still using 640x480 or smaller.
Then as windows proliferated on large screens, tiling has made a comeback. Finally enough space for those complicated screens seen in the movies...
Apple is leaving overlapping windows too.. overlapping windows are a pain in the butt, unless you really don't have enough space. It'd probably be a better thing to have windows scaled down when you need the space.
Windows also shouldn't be too big. Compare it to a sheet of paper which suddenly is way too big. Too much empty space. This is also why in I usually have the filetree / navigator bar open in any IDE.. you need some margin from the edge of the screen.
There is no one window arrangement strategy that works for you in all circumstances or for all people. Tiling can be useful in some use cases but there are times when overlapping or free-form or full-screen are more useful. I would not want a window manager that forced me into just overlapping or just tiling. That would be a failure. A good tool would give you a way to arrange in one style but to switch to another as needed.
I don’t see that Apple is leaving overlapping windows. They have introduced Stage Manager as one more window management system added to the others in their OS. It is not clear if people will take to it but it seems unlikely that everyone will or that it will become the only method. I do wish that Apple would strengthen their basic window management tools. The options on the green button are too limited.
