How to blur what's behind a Compose dialog, down to API 23

[timkrest]

Discussion for the article: https://github.com/timkrest/Underlay/blob/main/docs/article-window-boundary.md

It ends by asking what your device did: your minSdk, what mBlurEnabled said, and what onSourceChange reported. Answers go here. So do objections: the sample app shows which step won on your phone, and adb shell dumpsys window | grep mBlurEnabled says whether the system was ever going to blur.