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The behavior of qt-update-notifier is different than that by simple-update-notifier.

First of all, it waits 5 minutes to signal that there are updates, the icon from green with check turns orange with exclamation mark, clicking on it with the right mouse opens a drop-down menu (it is not possible to make updates from there but you still need to open synaptic which remains the “prince” tool for managing packages).

However you choose “open synaptic”, then you need to reload the cache (reload), click on updates→mark all updates→apply.

Also from the drop-down menu it is possible to check the log of the updates, practically synaptic history, silence qt-update-notifier or even mark that the updates have been done… until the next reboot, of course.

The good news is that it's less invasive compared to simple-update-notifier, it operates daily and not every x minutes, or hours.

Possibly always from the drop-down menu it is possible force control manually, more useful for developers than for individual users.