I recently upgraded to Big Sur and had the same experience while attempting to use the older, widget-style Pandora One app. No errors when opening the app, music starts playing, but no window was being rendered. While this older app may no longer be supported by Pandora, I was able to get everything working again by installing the latest Adobe AIR runtime; version 126.96.36.1995 at the time of this writing. And this is on macOS 11.2.3 as well. You can find the latest Adobe AIR runtime at the following link: https://airsdk.harman.com/runtime Hope this helps!!
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