I think the notification is triggered after connecting an external monitor to the laptop, and the notification persists until being manually closed.

And after unplugging the display cable, there's no active display, so DisplayManager [1] will not do configuration, which is intended. And ScreenLayoutObserver [2] will not be notified to update notification.

After re-opening the laptop, the display will restore its original state before sleep. Notification will not be updated because display count is not changed [3] (Before sleep only external monitor is connected, after resume only internal display is connected). In this case, actually we are not sure whether to show adding or removing notification.

[1] https://cs.chromium.org/chromium/src/ui/display/manager/display_manager.cc?q=display_manager.cc&dr&l=768
[2] https://cs.chromium.org/chromium/src/ash/system/screen_layout_observer.cc?l=386
[3] https://cs.chromium.org/chromium/src/ash/system/screen_layout_observer.cc?l=252

please remove the notification or allow disabling. I don't need a notification every time I plug/unplug extended monitors. My entire office uses extended screens and we hate this notification. If it was a notification toast that went away, fine. But this requires me to close it each time. And even when I unplug the monitor and close my chromebook, the notification is still there when I open and requires me to close it. It is worthless notification.

Re #2, Thanks for the feedback. Currently the notification persist until manually being closed, we might need a timeout. We need UX input for how long this timeout should be.

Re #3, A very short toast would be preferred. very short. 1-5sec 

I find this annoying myself. Feel free to assign this to me if you can not find another owner, I'll try to squeeze it in for 70.

Specifically, we should automatically dismiss any notifications that are no longer relevant, e.g.:
1. Plug in an external monitor; 'extending' notification occurs.
2. Close the device lid. Display is no longer extended and notification should be dismissed.

yes! Thank you so much. Someone in my office just complained about this to me the other day...lol

Thanks so much for taking this, I might not have bandwidth for M70.

IIRC, display notifiaiton used to be ephemeral. This was probably regressed when
we merged two notification systems.

Please make sure not to change "native resolution change" notification though, as it has UI to confirm the resolution change.

I think that this is covered by issue 827409.