The CyanogenMod team has uncovered a dormant feature existing in AOSP code called 'Heads Up' that is a notification mode that lets you preview and interact with “floating” alerts.
Heads Up shows an actionable notification card on top of the app you were running at the moment. The mode shows the notification cards you would see by swiping down the notification shade. Users can interact with the cards in the same way you would with notifications in the dropdown – you can swipe to dismiss or extend the card for further actions.
The 'Heads Up' feature has been present in the open source code of Android for a while, but was recently discovered by the CyanogenMod team. The feature is now a part of CyanogenMod 11 nightlies. The feature is available for dozen of devices listed here. The Heads Up feature was inactive and its possible that Google wanted to enable it in the next major version of Android.
The CyanogenMod team has added a Do Not Disturb function to the feature that lets you select specific apps that you don’t want interrupted by notifications. Users can selectively disable Heads Up if they don’t want any notifications showing on top of their video or game.
CyanogenMod had released the latest version of the ROM called CM11 M7 last month. The new iteration is based on Android 4.4.2 and brings new features like the much anticipated Theme Chooser UI that lets users customize each feature of their Android devices.
Source: CyanogenMod