KaOS uses "setuid" Bubblewrap instead of "unprivileged" Bubblewrap by default. This breaks Steam's ability to launch Windows games in Wine/Proton. Apparently it breaks Chromium-based web browsers too. As a workaround, you have to manually set it to unprivileged mode, which has to be done again if Bubblewrap is updated. https://github.com/flatpak/flatpak/wiki/User-namespace-requirements

If you find packaging issues (like in this case), bug reports are the best place to report. Even better would be a PR with the needed fix 🙂
Kernel is already build with the needed settings, so bubblewrap will be adjusted.
Without reports, it is hard to know of bugs (or fix them), personally not a user of Flatpak.

https://kaosx.us/docs/bugs/
https://github.com/KaOSx/main/issues

    demm Sorry, I will post it on GitHub too for organization’s sake.