ChromeOS has several types of system status and some of them should be notified to the user. This document summarizes which types of notifications are currently used and what kind of UI flow is necessary here.
The notifications are usually generated through the internal C++ API for the desktop notifications, which will appear as the rich notification as well.
Sometimes normal notifications should be blocked due to the current system status. For example, all notifications should be queued during the lock screen, and they should appear when unlocked. It also should be blocked when fullscreen state; notifications should be annoying when the user watches YouTube videos, or has a presentation (note: the notifications should appear in immersive fullscreen mode instead, because the shelf / message center UI is visible in such case).
However, the system status could be so important that they should appear in such cases. This does not mean all system notifications should appear; some could be misleading. For example, the notification for taking screenshot would behave as same as the normal notifications, so it should not appear in the lock screen. In addition, screenshot notification has the message to ‘click to view’ but click should not work in lock screen. On the other hand, connecting display would be okay to be notified even in the lock screen, it just notifies the new system status.
See also the notification blocking code update design doc.