Why does my screen show a black/white box on the left side?

When I was using the iMac today, I noticed development of this slowly over time. At first, there was a flickering of the image on the left side of the screen, loss of detail and shadows on things like the menu bar, windows, and color problems in the rest of the screen. Then, after some time, the region on the left turned black. Every minute or so, it would come back, only to slowly fade to the black box. Sometimes it would gradually fade to white before reverting to black. Now, after half an hour, it seems stuck with the same malfunctioning region being all white, whenever the screen is on, and the colors on the rest of the screen seem to be over-saturated. (See picture for more detail)

I have done some troubleshooting on my part, and have tried several things:

• Booting into safe mode: this graphical bug still happens, still white.
• Apple hardware test: same thing, bug still happens, still white, and no problems detected

Because of this testing, I believe I can safely rule out it being a graphics card failure, because if it were, the entire screen would be frozen, not just one column (according to the various forums I browsed looking for a solution).

From this point, I think it could be one of two problems, being either the power supply I replaced (supposedly it was a working, tested and refurbished PS, which was identical to the one I replaced even to the EMMC, and I got it from eBay), or the screen LCD, which may now be failing after ~15 years of service.

If anyone has had this problem, and has fixed it, please let me know how you did, or at the least, what the source of the problem was.