MacOS – Web clip widget in Notification Center

I'm a Mac OS X developer and I just finished adding Notification Center support to my app. For what it's worth, here's what I found out:

• By default, notifications (the alerts) are NOT shown if the application is already frontmost (this explains why alerts refuse to show up sometimes, and you only see the notification in Notification Center). This is up to given applications to overwrite and it is not very straightforward, so do expect most applications to not show notifications when they're active. In my opinion, this is very annoying.
• The application, after sending a notification still has a lot of control over that given notification. It can remove it from your screen at any time it wants, regardless of the settings you have. Basically, if Safari, for some reason was set to remove notifications after X seconds, it will do that, your settings do not matter.

I have no idea if this is a bug in Safari or Notification Center, or both. However, I am fairly confident that there is nothing you can do about the second issue (there might be a hidden defaults setting for the first one, but I haven't heard of it). I did check the WebKit API Specification, to make sure it's not something that the individual websites are doing, and you can rule out websites from your suspect's list, because they do not have any control over the notifications.

I guess we'll have to wait and see if Apple will address this in its next bug fix release (10.9). However, by the looks of it, not showing notifications when the app is frontmost is a "feature", so don't get your hopes up.

The notifications that you receive for Mail within Notification Center are local notifications, not push notifications from some Apple server. As such, the program has to be running in order for the action that will trigger a notification to be able to do so.

Notification Center cannot check for new mails by itself, it has no in-built capabilities for polling/checking etc in order to create notifications, it merely receives them and displays them.

It is possible that in due course some email app will be made available that supports push notifications, but at present none do, including Apple's own. The fact that you get such notifications on iOS is more a feature that the mail client is always running in order to check for new mail, and create the local notifications accordingly. Leaving mail open all the time on your Mac isn't really an issue, it's hardly resource intensive, and if upon receiving a mail alert you wish to act upon it, then opening the alert will only open mail for you anyway.

The best way to summarise it is that NC just listens for alerts that other applications create, and displays them according to your preferences. It takes no part in the creation of alerts.