I have also seen some apps end up in completely arbitrary (inaccessible) places for unknown reasons. That's true under winex11, too, but there they seem to eventually get moved to more reasonable positions. Perhaps the X server plays a role here, or something else is going on, and I need to investigate further in case there is something we can replicate.
Yes, I'm not 100% sure and I don't remember the details but I believe there are similar mechanisms in both Windows and X window managers, automatically adjusting window positions when they are off. In both cases I think it's not a hard requirement, and some applications manage to override it. However this also means that some other applications may expect the mechanisms to be there and to adjust their windows sizes to the right position and size.
With this in mind, the geometry adjustment and fullscreen window resize prevention look like workarounds for the cases where applications expect to succeed overriding the constraints, but it also looks to me that they may go against that mechanism and prevent a bit too much resizes from happening, and break the applications that may be expecting them.
Anyway, I'm also not completely opposed to having it at this point, and the MR looks mostly good now, so I could probably approve it with the `window_rect` comment above addressed.