In 16.04 when there was a calendar reminder for an event in Thunderbird/Lightning it would give a system-wide pop up reminder. These would appear at the forefront of the screen no matter what application was currently being used, even in Thunderbird was minimized or right after waking the computer from sleep. Now in 18.04, if Thunderbird is minimized these notification popups do not appear. Instead, I have to go to the sidebar and I can see the notification window when I move the cursor over the thunderbird icon.
Why do these notifications no longer appear at the front of the screen? Is there any way to change this behavior in 18.04? I tested Evolution and the calendar alerts there are system-wide pop-ups, as Thunderbird was in 16.04.