I have a late 2012 27" iMac with macOS Catalina. Recently I replaced it's hard disk with an SSD and I was extremely happy with the results. However, the replacement required removing the display from the chassis & putting it back with some adhesive strips. The strips I used apparently was not strong enough. A few weeks after doing the replacement, one morning I woke up with the display glass came off of the chassis and dropped onto the keyboard on my desk, pulling its power & data cables out of the board.
I plugged the cables back and reinstalled the screen. At the first boot it started with a "snowy" screen like this What causes “snowy” display on iMac screen after replacement (ifixit.com) And since then, the computers boots only at 1280x720. I bought a new data cable and replaced it, nothing has changed. I tried resetting PRAM & SMC, I booted to ubuntu using a flash disk. None of it works.
It is an extremely weird situation. The display works. I don't have any dead pixels or anything. But the resolution is terrible. Does anyone have any idea?