This app was mentioned in 13 comments, with an average of 1.85 upvotes
As standard it only allows for split view multitasking with 2 adjustments to the size of each window. There are apps available which allow for pop-up window multitasking such as this https://play.google.com/store/apps/details?id=com.lwi.android.flappsfull although this is not proper windowed multitasking.
I believe Spotify doesn't offer PIP even in >Android 8 but you could try launching through the web player which I believe does, at least in Windows.
You could also have a play with Floating Apps and loading up your web playlist.
Alternatively and probably most eloquently, you could create your own UI using AutoTools, Material Design Plug-in, Tasker Scenes etc. You may find it useful over here.
I got Floating Apps and it works perfectly. Just like the Samsung pop up windows.
There is a free version, but I highly recommend getting the paid version as there's some awesome features.
Might I recommend "Floating Apps"? https://play.google.com/store/apps/details?id=com.lwi.android.flappsfull Yes it's holo. Yes it hasn't been updated in over a year.
However, with its ability to make Any widget or Web page into a custom floating app, I now can have floating windows (and Multi-Window. It supports that too) for Google Maps, Google Keep, reddit, Dropbox, YouTube, facebook, and many others.
Well floating window is an easy fix : https://play.google.com/store/apps/details?id=com.lwi.android.flappsfull
I wish I had a full screen split keyboard, but I'm sure a solution for that will come soon enough.
I just played around with Floating Apps yesterday again. It also has a free version, a docs addon, and a PDF addon. It didn't fit my needs so I uninstalled it, but maybe you'll like it.
Also there's this one that I ran across just now, but I haven't tried it yet.
Folosesc Floating Apps cu youtube in fundal.
https://play.google.com/store/apps/details?id=com.lwi.android.flappsfull
link to the paid version. there is another version that's free but I haven't use the free version for a very long time
I wonder if this will help(can view but they have a free version as well): https://play.google.com/store/apps/details?id=com.lwi.android.flappsfull
https://play.google.com/store/apps/details?id=com.lwi.android.flappsfull
There is a free one I think