As one of the most popular and fully-featured video player apps available on the Play Store, we've become accustomed to MX Player getting useful new features on a regular basis. Most recently, the team behind it added Chromecast support for online streaming content, and the latest update includes a Picture-in-Picture function.
Version 1.11.3 is rolling out to users now, and it brings very in-vogue feature in PiP. You can enable it by navigating to the player options once you've got a video running (the little arrow by the screen rotation button in the top left of the screen) and selecting the little PiP icon. You'll then immediately enter PiP while staying in the app and returning to the video list, but you can then go home or open any other app and the video will keep playing over the top in its little window.
The PiP frame will snap to whichever side of the screen you choose, and there's a button in the center that takes you back into full-screen mode. You also get skip forward, back, and pause controls in the expanded PiP window, as well as an X to quit it altogether, but there doesn't seem to be a way to resize it. The settings cog in the corner takes you to the Android settings toggle that allows the app to go full-screen rather than the app's own settings, curiously.
Now Watch while you Browse! Introducing Picture-in-Picture Mode (Android 8+) that allows you to continue watching your local videos even when outside the app.
According to the changelog (above), PiP in MX Player will work on phones running Android 8.0 Oreo and above. That's all there is to the latest version of MX Player. If it's not rolled out to you via the Play Store already, you can grab the 1.11.3 update from APK Mirror.