Why did my screen develop a darker area on the bottom edge?

My T430s worked perfectly for 10 years, but suddenly developed a darker area on the bottom edge when I turned it on one day. At first, I thought it was a broken backlight problem, but the darker area sometimes expanded to a wider area along the bottom edge after a power and lid cycle, and even more occasionally, it could disappear, when the problem first occurred. Nowadays, this darker triangle remains pretty stable.

The laptop had been hit or dropped, and has been used as a desktop (less mobile) for work. A friend at Lenovo suggested reinstalling the graphics driver, and I did, but the problem persisted.

As this laptop is still strong and perfect for my use cases (coding projects and online video streaming), especially after I upgraded the RAM and SSD a couple of years ago, I'd like to fix it if it's just a cable problem or so. What's the most likely cause for this? If you could kindly highlight the potentially broken parts (better with a name and picture) and guides for replacement/repair, I'll be very grateful.