In the mid-90s I used to repair PCs. Customer brought PC in where left mouse button did not function. Easy. Replace mouse. NOPE. OK, software issue. Reinstall

In the mid-90s I used to repair PCs. Customer brought PC in where left mouse button did not function. Easy. Replace mouse. NOPE. OK, software issue. Reinstall mouse driver. NOPE. OK, deeper software issue. Replace HDD from working PC. NOPE. OK, replace RAM? NOPE. OK, replace motherboard and all add-in cards. NOPE. At this point we have a different HDD, motherboard, CPU, RAM, video card and mouse. Still left mouse button doesn't work. Mouse moves fine. Right button works. Only thing left is the case and the PSU. Replace PSU. Left mouse button works perfectly. FML.