I haven't managed to fix the GUI, but I did manage to fix the rest of the dysfunction by manually editing `mimeapps.list` and running `kbuildsycoca5`.
My best guess is that, when the value in lines like `x-scheme-handler/http=firefox.desktop;` doesn't exist, KDE synthesizes a fallback chain, and kde-open5 and xdg-desktop-portal-kde disagreed on the ordering of the synthesized fallback chain.
The GUI still resets to like in the screenshot though.
I haven't managed to fix the GUI, but I did manage to fix the rest of the dysfunction by manually editing `mimeapps.list` and running `kbuildsycoca5`.
My best guess is that, when the value in lines like `x-scheme- handler/ http=firefox. desktop; ` doesn't exist, KDE synthesizes a fallback chain, and kde-open5 and xdg-desktop- portal- kde disagreed on the ordering of the synthesized fallback chain.
The GUI still resets to like in the screenshot though.