I’ve been building a window manger for macOS that involves hiding windows offscreen, but I’ve found AccessibilityKit clamps position writes at the screen edge, so I’ve had to settle for allowing a 1px-wide sliver (plus shadow) in the corner. Is that what you’re doing too, or did you find an alternative?
Same issue — I never found a way around the clamp. I park the window's top-left at (maxX - 1, maxY - 1) of the union of all displays' CGDisplayBounds, i.e. just inside the bottom-right corner of the whole arrangement. The clamp is much looser going off the bottom/right than up/left — pushing to something like (-30000, -30000) gets snapped back near the top-left and leaves a full-height sliver, whereas bottom-right leaves just the corner pixel (plus its shadow, which I also haven't beaten).
Since the target is derived from the union, a display disconnect or resolution change shrinks the desktop and the parked window pops back into view. I listen for display-config changes and re-apply the position.
Thanks for the corroboration. Something that apparently might be possible is using the private framework for Spaces to offload windows to a "holding Space"; I haven't tried it because it felt just a little too hacky for something I'm ideally hoping to turn into a commercial product, but it might be worth investigating depending on your use case.