Using Gramps 5.1.3 on Ubuntu 21.04 (Wayland), I’m finding that Gramps is not remembering window positions at all. If I close and re-open a window, it always reopens at 26 pixels from the left, 23 pixels from the top.
It is a design decision in Wayland/desktop to not expose absolute window positions to clients at all. This means that you simply cannot know where a top-level window is precisely, you can only know which outputs it overlaps with.