This is a weird problem that some people have reported as far back as 4+ years! Looks like maybe 10 different people maximum that reported and received no useful responses. Many more responded with "I also have this problem" which doesn't mean they have exact problem or just interested. So, I call it very few people and low, low priority.
However, that doesn't make those, including my visually impaired client, very happy! So, I am doing my best to find the common link/cause of the problem and some resolution it, or at least a way to stop the infinite filling.
Problem: In ANY input field. i.e., Browser fields (URL field, Google.com field), some forms, word processor, and even Window Sign-in before Windows even active have resulted in some character, in our case "-" or +l" or "/" or "0" or etc to just on its own start filling the field and beyond. You will see, e.g., AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA ... forever. We initiall had no way to stop it and couldn't type what we wanted. Recently, if you catch it, maybe early enough, using the Backspace Key to continually erase them and typing a quick key to insert something else will stop it and life can continue. Up to this discovery, the only way to clear it was to reboot. But, then we one time hit it at Windows Signin! So, RESTART and do it again until clear.
Things tried and problem still persisted:
Changed Keyboards
Removed Keyboard
Windows SAFE mode
Updated all Keyboard Drivers
Ran all HP Hardware Tests successfully
Ran HP BIOS update check -- Back Level was updated just an hour ago. Hopefully this might work, but after it PASSED the date given was 3/8/22 for F.22 level and this is exactly what we originally had. So, the HP diagnostics either reported that we were back level, or the PASSED report reported it wrong. Next time over there, I'll run SPECCY to see if it reports a newer date.
Update 1/18/24. HP should have told me to RESTART! This morning I checked again using "Run MSINFO32.exe" and it still said F22. Checked HP site again (no help), except set up HP ID. The downloaded code to look at the machine did not work. Even after setting up an account ID it didn't help. Did a RESTART as seen needed under Device Manager>Drivers. It updated the BIOS to F40 7/17/23. Short of current F42 as per support system. We'll run with this to see if problem reoccurs. Apparently, the caretaker said it just started happening when they went to new keyboard. Why? Because the old Visually Impaired Keyboard was reported as not functioning (turned out it was jammed in port next to the USB that it should have gone into. I believe this is for the printer cable!) Maybe this jam caused the problem with the now possibly damaged port, whatever it is, OR the keyboard cable connector). They went to a wireless, but it wasn't a Visually Impaired one and useless to him. I went over and plugged the old keyboard back in, but by this time they had already ordered another Visually Impaired one (Corded). That one is now active and no problems yet on it. Time will tell on the new BIOS code and new keyboard. If anything the problem is with the USB/Keyboard to USB.
Did SFC /SCANNOW -- no issues
Checked Event Log, but had no date/time to see if anything was reported during error -- nothing found to date
Additional Information:
Over the 4+ years of issue reported, there is no common PC brand or type. More than HP had problem, Ours is Desktop and Laptops with their fixed keyboards also had problem. One individual went to the trouble of disconnecting the laptop keyboard from the motherboard and installing an external one. There were some differences as to some keys working or not working, but the issue persisted.
I'll be adding to this post as we test more, get this corrected, or find a reasonable workaround.
---------------
1/18/24. Restart done to install F40 BIOS. Awaiting any error reports.
--------------------
3/2/24. https://answers.microsoft.com/en-us/windows/forum/all/character-repeats-automatically-forever-in-all/59fcc9ce-98ce-4740-b539-ddd64d66477e details the Repeating Character troubleshooting and resolution as of this day. Bottom Line it seems like an old surge protector, one I can no longer find on the Internet either didn't stop an electrical surge it itself had a problem that sent a charge to the PC. A full memory test via Memtest86 passed, leaning us with most likely a motherboard failure, or just the surge protector failing. We have run a week without the problem appearing after a new surge protector (about 4300 joule one) replaced the old one. Regardless, a new PC was bought and the old one will go to a friend. Many reports of this problem appear on the Internet with not one resolution or root cause. This could be it!