Thanks so much for your reply. Byword actually does this the same way as TextNut, but the following apps do it the way I am mentioning when scrolling with the keyboard:
- IA Writer
Here's a video demonstration: https://drive.google.com/open?id=0ByJLgLsrE1H-S3YwY0hMTnhLaUE
I agree that the mouse doesn't need to do this. Out of all the apps I tried, only one app implements this for mouse scrolling and that is Ulysses.
What it does is moves the cursor up and down when you scroll with the mouse.
Other apps like Byword, Write, Focused and IA Writer scroll normally with the mouse (similar to TextNut) and only lock back into the center when you use the arrow keys or type a word with the keyboard.
Perhaps try out the trial of Ulysses to see if you like the behaviour with the mouse. Personally, I don't really mind so much what the mouse does, but the way Ulysses has implemented it is pretty cool.
Really appreciate your help and really can't wait to see what you do with this great app