Taskbar, a utility that turns your Android into a desktop PC, has been updated to version 4.0, and now devices running the newest version of Android can take advantage of its freeform windows feature. The feature did not work with Pie at first because the new OS had completely done away with the calls previously used to invoke freeform window mode. XDA Developers member and Taskbar developer farmerbb dove into Android 9 Pie's code and found that freeform windows were still available, but were now part of a per-app window type calling feature. He routed Taskbar's call to this new Android feature, and it worked like a charm.
There are a few side notes and caveats with this new approach to freeform windows. One cool new feature in Google's windowing API is that freeform windows can now float on top of fullscreen windows, where before you could call up multiple freeform windows, but they could only sit over a homescreen or a blank activity. You also no longer need to enable freeform window functionality via an ADB command, though that may not last; the app works around certain rules about what Play Store apps can and can't do by targeting an older API version for Android, being version 27. Eventually, Google will force the app to transition to version 28 or get out of the Play Store, and that will break the current way of doing things. There is a workaround planned for that time, but there's no way of knowing for sure if it will work until the time comes. The app also doesn't work with YouTube, since it uses the same native Android windowing call that YouTube picture-in-picture uses. As such, if you don't have YouTube picture-in-picture in your region, don't expect this app to give it to you.
Taskbar is one of the more advanced desktop-style apps out there, and it can even be set as your homescreen in order to avoid clutter. It can use icon sets in its taskbar strip and mobile app drawer, and of course, can generate and move freeform windows to give you a desktop-like multitasking experience.