Consider the following text:
Hello my name is Tom And this is
Now, put the cursor at the beginning of "n" and hit the down key on the keyboard twice.
It should now point to the "is" but instead it's pointing at the "this". This works correctly on other platforms, just not in textblock. It's a long-standing bug that should be fairly easy to fix (just keep visual position of the cursor when jumping up/down and clear it otherwise).