It is not restricted to any browser. Happens in terminal, application (start) menu search box, anywhere there is text input. Normally, if you are typing, you can type with shift held, and it is ignored when you press space. This is no longer the case.
Installed from the AUR using pamac: https://aur.archlinux.org/packages/zoom (and removed - currently the flatpak is installed… I’ll be adding another issue there, for the 2 packages that are permanently showing up in the updates since then, org.freedesktop.Platform and org.kde.Platform - I only mention it here in case it is relevant)
Around the same time, (I didn’t mention it earlier, because I thought that I did install it a week earlier, but now I’m not so sure) I tried to install a keyboard layout viewer (since I installed a foreign keyboard layout and wanted an onscreen visual of the layout) I believe the package was kcm-fcitx - I thought I had removed all traces of it, but I can’t be sure. Which is why I was asking if there was a way of tracking which programs have stolen the keyboard hooks.
I have been going through all the dependencies, and I think they are all removed except the ones that are required by kolourpaint and mpv. Although I wonder if libxkbcommon could also have something to do with it.