tdd-tetris-tutorial | You make a Tetris game | Unit Testing library

I've been applying TDD to some new projects to get the hang of things, and I understand the basic flow: Write tests that fail, write code to pass tests, refactor if needed, repeat.

To keep the tests fast and decoupled, I abstract out things like network requests and file I/O. These are usually abstracted into interfaces which get passed through using dependency injection.

Usually development is very smooth until the end where I realize I need to implement these abstract interfaces. The whole point of abstracting them was to make it easily testable, but following TDD I would need to write a test before writing the implementation code, correct?

For example, I was looking at the tdd-tetris-tutorial https://github.com/luontola/tdd-tetris-tutorial/tree/tutorial/src/test/java/tetris. If I wanted to add the ability to play with a keyboard, I would abstract away basic controls into methods inside a Controller class, like "rotateBlock", "moveLeft", etc. that could be tested.

But at the end I would need to add some logic to detect keystrokes from the keyboard when implementing a controller class. How would one write a test to implement that?

Perhaps some things just can't be tested, and reaching 100% code coverage is impossible for some cases?