With the most recent release of #NewPipe (0.20.0) I've spontaneously started picking it over the official #YouTube #Android app for playback. It's just that good now. Automatic transition to background/windowed playback when switching from the app *rocks*!
It's not a completely seamless transition yet, there's a noticeable hiccup when that happens, but I have faith in #opensource community.
Find it on @fdroidorg here: https://f-droid.org/packages/org.schabi.newpipe