The same thing happens in Chrome OS. It would be nice if the notification remained in the notification area until cleared.

Test

syncing outlook tasks with toodledo task and adding a link to outlook task would be one way of solving this - then the outlook task would appear in calendar inlcuding notification and clicking on toodledo link would get into full info about task. Or just tick off from within calendar app and both Outlook task and Toodledo task would be ticked off.

Ideally such notifications could also be optionally via mobile phone (e.g. IOS) notification system, text message, and email.

Basic would be to see one communication channel selected per user account as the chosen way to communicate such notifications.

Even better would be to be able to select an account-wide default but also have the option to change the notification channel selection for a given task. For example, I might want to usually get notifications on my phone, but for certain tasks, I might want to get an email.