The popup width is fixed, so don't rely on the actual dialog window width which might be bollocks.
More importantly, place the popups immediately and before they are shown so they show up correctly and don't fly all over the place.
It can happen that the popup contents aren't readily laid out when we enter positionPopups() after object creation, so don't reserve any space for the popup until it

The first popup might still fly in from outside the screen or something but it's a significant improvement on wayland at least. For some more elaborate notifications I now have a slight delay before they show up but that might have been also one of the reasons for them showing up in the wrong place before.

Please try triggering alternate notifications from [1] and notify-send. It's very strange why sometimes you have a delay now but at least stuff doesn't fly all over or cause weird gaps :)

(Don't really know which bugreport on bugzilla is that particular issue, there's some others with notification popups overlapping and appearing in random places but might be unrelated)

[1] https://tests.peter.sh/notification-generator/