It turns out Ubuntu 18.04 with GNOME 3.28.4 supports unread notifications indicator (badge) but very few applications make use of it properly.
Some apps don't make use of it at all. Some have bugs that prevent it from working. Some try to make use of it but it only work if installed in a certain way.
Mailspring supports it but it doesn't work if it's installed through the Snap store. If you install it with the .deb package you can get it to work but there's this bug from 2017 that prevents it from working out of the box. To get it fixed, you need to rename /usr/share/applications/mailspring.desktop to Mailspring.desktop with a capital letter. Then press Alt+F2 and execute command r to reload the shell.
Some other apps such as Thunderbird and Firefox have had support for it in the past via extensions, but at the moment of writing both of them are broken as well. Here's the Thunderbird issue, you might be able to get it working by installing thunderbird-gnome-support package from apt.
If your apps don't show notification indicators, you can try:
Searching if there's a workaround for the specific app you're
having problems with.
Installing the app from a different source
(haven't seen any Snap app support badges)
Send a feature request / bug report to the developers of your specific app
Here are some related discussions I found while investigating the issue:
It turns out Ubuntu 18.04 with GNOME 3.28.4 supports unread notifications indicator (badge) but very few applications make use of it properly.
Some apps don't make use of it at all. Some have bugs that prevent it from working. Some try to make use of it but it only work if installed in a certain way.
Mailspring supports it but it doesn't work if it's installed through the Snap store. If you install it with the .deb package you can get it to work but there's this bug from 2017 that prevents it from working out of the box. To get it fixed, you need to rename
/usr/share/applications/mailspring.desktop
toMailspring.desktop
with a capital letter. Then press Alt+F2 and execute commandr
to reload the shell.Some other apps such as Thunderbird and Firefox have had support for it in the past via extensions, but at the moment of writing both of them are broken as well. Here's the Thunderbird issue, you might be able to get it working by installing
thunderbird-gnome-support
package from apt.If your apps don't show notification indicators, you can try:
Here are some related discussions I found while investigating the issue: