Just helping Google out here: the notification just says “Signal(1) is ready”, which is caused by Electron lacking support for Gnome’s focus-stealing prevention feature.
This can be worked around with the Window Is Ready – Notification Remover extension. As of this writing, I have yet to determine whether the extension then also restores the (annoying) focus-stealing ability, or if only the notification is removed (which would leave focus-stealing windows in the dark).