Notification spec says, when replacing a notification:

The server must atomically (ie with no flicker or other visual cues) replace the given notification with this one.

Notifications shifting about is a "visual cue".

I've noticed that with D29770 where I always update notification text, notification can randomly swap places depending on which free space job happened to be finished first.

Since this is a "fix to make it more spec compliant", could go into 5.18? Definitely should go into 5.19