You know how people like 'stock' Android for its lack of bloatware and overall streamlined experience? Well, the flipside to that is that some actually useful features are missing from the Android that Google puts out. Some makers add extra bits on top of it into their custom overlays and years down the road those features make it to Android natively. Such was the case, for example, with multi-window multi-tasking - it was introduced on the Samsung Galaxy Note 3 back in 2013 (running Android 4.3 Jelly Bean), but it wasn't until Android 7 Nougat in 2016 that such a feature was implemented...
Source: https://www.gsmarena.com/scrolling_screenshots_now_planned_for_android_r-news-37105.php
