Show critical notifications on all screens

Feature summary

When using a multi-monitor setup, notifications with priority "critical" should be displayed on all screens.

Relevant links, screenshots, screencasts etc.

First note that I am still using Gnome 3.38.6 on Debian old-stable, and could not test with a newer version: please apologize if my issue is not relevant on recent Gnome.

"critical" notifications are often "important", but also "urgent". The example I have in mind is when doing some presentation (say with LibreOffice Impress, or some PDF in full-screen) with the computer plugged to a video-projector: currently, when battery is low, notification only appears on the primary screen (probably the laptop's one) so it is easy to miss it. Same thing happens when watching a movie: you stare at the big screen / video-projector, and don't look the other screen.

You can argue that in these situations user should probably switch off the screen that is not looked at. Taking again the example of the presentation, is see two issues:

• when installing everything for a presentation, you have many other important things to think of
• when doing the presentation you often need both screen, and you can spend a few minutes answering questions and during this time you don't look regularly both screen.

And I suppose people using three monitors are even more likely to miss those critical notifications.

Regards, Yvan