Transient notifications are removed after being shown. If the summary
is being shown while they appear, they are represented in it by a new
source icon.
We always create a new source for new transient notifications to
ensure that they don't replace the latest persistent notification
associated with the source. Because we generally don't want any
new or resident notifications to be replaced by others, associating
multiple notifications with a source is the next thing we will
implement.
https://bugzilla.gnome.org/show_bug.cgi?id=633412
22f4aabadf