HeadsUp will inform you about new notifications by showing a little popup window in top of the screen.

HeadsUp is a fork of AcDisplay mainly focused on displaying notifications while your device is on. It will inform you about new notifications, while you’re browsing/gaming/watching movies, by showing a minimal, beautiful popup, allowing you to open them directly or perform an action.

Note: This app is mainly for Android versions ≤ 5. It might work on newer versions (reportedly does on Lineage