At least everyone knows that they should never buy a casio CFX-9850GB plus with horizontal missing lines.

Or just cope with it.

^This.

I'm gonna reuse the shell and keyboard and build something else in it probably

Ooh, sounds neat!

Ouch, what's the state of the screen now? From what it sounds, desoldering did nothing to make it better (might have made the screen even worse!)

Btw I have the exact model Casio CFX-9850GB+ (white version).
One of my few graphing calculators.

Ouch, what's the state of the screen now? From what it sounds, desoldering did nothing to make it better (might have made the screen even worse!)

Btw I have the exact model Casio CFX-9850GB+ (white version).
One of my few graphing calculators.

Ribbon is destroyed. Screen entirely dead.

It sucks how they did it, preventing you from fixing the calc. Otherwise with a normal screen connector apparently it's really easy to fix. Is it true that sometimes it's just because the connection is dirty?

The connection cannot be dirty because the ribbon cable is soldered directly to the board, but the solder can dry a bit and crack thus making a bad connection.

So when you get a calc and see there are horizontal lines missing on the screen: it's not an easy fix. It is impossible to service with a normal soldering iron. Do not attempt to fix it because it will definitely brick your calculator.