@philjcrawford I moved out of Opera and now use Vivaldi as my main browser, but just would like to add that until a few years ago, before I tried Opera again (I'd been a user since before the Chromium take over, but switched a long time ago and tried again recently), I used to use Yandex as my main browser and its Linux version does support PWA and the PWA detached window can be reached both from the OS like an app and from its sidebar. You can set it to show up like the way Edge and Opera do, but you can make it fully detach as a separate window directly from the browser's sidebar. Yandex has a more stable version on Windows though. I think the Linux version is still in beta.
So yes, this is feasible in other OSs too.
I would just like to add that Yandex was born from Opera a long time ago. It's originally a fork from Chromium Opera that improved about every single trademark feature from it, including video popup, speed dial, sidebar, everything.
I moved out of it for a few reasons, including its UI started to take up too much space in the small monitor I had at work and I wanted to try Opera again. This was like a year or two before Opera's rebranding. Then came Opera's rebranding and a lot of stuff started breaking, so now I'm on Vivaldi which has its fair share of bugs, but has all the trademark Opera features + PWA support, which is probably the main reason I'm going to stick with it.
BTW, I didn't know until I moved to Vivaldi, but I think Vivaldi's team used to work for Opera in the old times, so you can see a lot of inspiration from the original Opera browser like being possible to choose where to put the tab bar and other customization options