A Bad HDMI Cable Broke My Mouse

I got tasked to fix some bizarre issues a friend was having with their gaming PC. They had had lots of bluetooth issues, and recently their mouse stopped working entirely in Windows 11 - nothing would show up in Control Panel. Other mice also didn’t work. It worked fine in the BIOS and in an Manjaro installer, so I got suspicious and for the first time in my storied career there was actually bad stick of RAM! This person had expressed interest in switching to Linux before, and they had done enough fiddling around with drivers (plus the OS was installed with bad ram I’m never trusting it again) that I convinced them to install Manjaro. The mouse worked fine in the installer, rebooted, no mouse. Or keyboard now! We made it worse! Ctrl + Alt + F1 switched to a working TTY though, which was odd. I tried updating the BIOS, checking dmesg. The chatbots had me going down wild goose chases in the BIOS fiddling IOMMU settings and XHCI handoffs. Finally I decided to just start unplugging things…